chore: Update discovery artifacts (#1410)

## Deleted keys were detected in the following stable discovery artifacts:
managedidentities v1 https://github.com/googleapis/google-api-python-client/commit/e96adbb1ba3e4e56d916cc28474f85543f17ad0e

## Discovery Artifact Change Summary:
feat(appengine): update the api https://github.com/googleapis/google-api-python-client/commit/ffcf86035a751e98a763c8a2d54b70d3a55ca14d
feat(containeranalysis): update the api https://github.com/googleapis/google-api-python-client/commit/9a1c70b7df3e074fc9fbd0eebdaf75a91046078c
feat(documentai): update the api https://github.com/googleapis/google-api-python-client/commit/07a6e774ac185442a99437896eaee774946b5846
feat(drive): update the api https://github.com/googleapis/google-api-python-client/commit/773910fdf25b084aa3623d24fe99c8a1330fbecb
feat(managedidentities): update the api https://github.com/googleapis/google-api-python-client/commit/e96adbb1ba3e4e56d916cc28474f85543f17ad0e
feat(spanner): update the api https://github.com/googleapis/google-api-python-client/commit/87da2f3605ec1b8986324cddc33f2b5601d3e896
diff --git a/docs/dyn/fcm_v1.projects.messages.html b/docs/dyn/fcm_v1.projects.messages.html
index bc04477..1e3d17a 100644
--- a/docs/dyn/fcm_v1.projects.messages.html
+++ b/docs/dyn/fcm_v1.projects.messages.html
@@ -157,15 +157,15 @@
         "analyticsLabel": "A String", # Label associated with the message's analytics data.
         "image": "A String", # Contains the URL of an image that is going to be displayed in a notification. If present, it will override google.firebase.fcm.v1.Notification.image.
       },
-      "headers": { # HTTP request headers defined in Apple Push Notification Service. Refer to [APNs request headers](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns) for supported headers, e.g. "apns-priority": "10".
+      "headers": { # HTTP request headers defined in Apple Push Notification Service. Refer to [APNs request headers](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns) for supported headers such as `apns-expiration` and `apns-priority`.
         "a_key": "A String",
       },
-      "payload": { # APNs payload as a JSON object, including both `aps` dictionary and custom payload. See [Payload Key Reference](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification). If present, it overrides google.firebase.fcm.v1.Notification.title and google.firebase.fcm.v1.Notification.body.
+      "payload": { # APNs payload as a JSON object, including both `aps` dictionary and custom payload. See [Payload Key Reference](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification). If present, it overrides google.firebase.fcm.v1.Notification.title and google.firebase.fcm.v1.Notification.body. The backend sets a default value for `apns-expiration` of 30 days and a default value for `apns-priority` of 10 if not explicitly set.
         "a_key": "", # Properties of the object.
       },
     },
     "condition": "A String", # Condition to send a message to, e.g. "'foo' in topics && 'bar' in topics".
-    "data": { # Input only. Arbitrary key/value payload. The key should not be a reserved word ("from", "message_type", or any word starting with "google" or "gcm").
+    "data": { # Input only. Arbitrary key/value payload, which must be UTF-8 encoded. The key should not be a reserved word ("from", "message_type", or any word starting with "google" or "gcm"). When sending payloads containing only data fields to iOS devices, only normal priority (`"apns-priority": "5"`) is allowed in [`ApnsConfig`](/docs/reference/fcm/rest/v1/projects.messages#apnsconfig).
       "a_key": "A String",
     },
     "fcmOptions": { # Platform independent options for features provided by the FCM SDKs. # Input only. Template for FCM SDK feature options to use across all platforms.
@@ -267,15 +267,15 @@
       "analyticsLabel": "A String", # Label associated with the message's analytics data.
       "image": "A String", # Contains the URL of an image that is going to be displayed in a notification. If present, it will override google.firebase.fcm.v1.Notification.image.
     },
-    "headers": { # HTTP request headers defined in Apple Push Notification Service. Refer to [APNs request headers](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns) for supported headers, e.g. "apns-priority": "10".
+    "headers": { # HTTP request headers defined in Apple Push Notification Service. Refer to [APNs request headers](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/sending_notification_requests_to_apns) for supported headers such as `apns-expiration` and `apns-priority`.
       "a_key": "A String",
     },
-    "payload": { # APNs payload as a JSON object, including both `aps` dictionary and custom payload. See [Payload Key Reference](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification). If present, it overrides google.firebase.fcm.v1.Notification.title and google.firebase.fcm.v1.Notification.body.
+    "payload": { # APNs payload as a JSON object, including both `aps` dictionary and custom payload. See [Payload Key Reference](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification). If present, it overrides google.firebase.fcm.v1.Notification.title and google.firebase.fcm.v1.Notification.body. The backend sets a default value for `apns-expiration` of 30 days and a default value for `apns-priority` of 10 if not explicitly set.
       "a_key": "", # Properties of the object.
     },
   },
   "condition": "A String", # Condition to send a message to, e.g. "'foo' in topics && 'bar' in topics".
-  "data": { # Input only. Arbitrary key/value payload. The key should not be a reserved word ("from", "message_type", or any word starting with "google" or "gcm").
+  "data": { # Input only. Arbitrary key/value payload, which must be UTF-8 encoded. The key should not be a reserved word ("from", "message_type", or any word starting with "google" or "gcm"). When sending payloads containing only data fields to iOS devices, only normal priority (`"apns-priority": "5"`) is allowed in [`ApnsConfig`](/docs/reference/fcm/rest/v1/projects.messages#apnsconfig).
     "a_key": "A String",
   },
   "fcmOptions": { # Platform independent options for features provided by the FCM SDKs. # Input only. Template for FCM SDK feature options to use across all platforms.