chore: Update discovery artifacts (#1310)

## Deleted keys were detected in the following pre-stable discovery artifacts:
alertcenterv1beta1[ [More details]](https://github.com/googleapis/google-api-python-client/commit/575006802e6b74ea6b64b5e43c7525cb9d528d38)

## Discovery Artifact Change Summary:
alertcenterv1beta1[ [More details]](https://github.com/googleapis/google-api-python-client/commit/575006802e6b74ea6b64b5e43c7525cb9d528d38)
diff --git a/googleapiclient/discovery_cache/documents/firestore.v1.json b/googleapiclient/discovery_cache/documents/firestore.v1.json
index e6d239b..1d16eb6 100644
--- a/googleapiclient/discovery_cache/documents/firestore.v1.json
+++ b/googleapiclient/discovery_cache/documents/firestore.v1.json
@@ -1135,7 +1135,7 @@
                   "type": "string"
                 },
                 "pageSize": {
-                  "description": "The maximum number of results to return. If not set, the service will select a default.",
+                  "description": "The maximum number of results to return. If not set, the service selects a default.",
                   "format": "int32",
                   "location": "query",
                   "type": "integer"
@@ -1160,7 +1160,7 @@
       }
     }
   },
-  "revision": "20210326",
+  "revision": "20210419",
   "rootUrl": "https://firestore.googleapis.com/",
   "schemas": {
     "ArrayValue": {