Update release notes files

Docs-Preview: https://skia.org/?cl=258417
Bug: skia:
Change-Id: I4716231ad260f2f7f29776f7cc455c6837ec6a43
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/258417
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Heather Miller <hcm@google.com>
Auto-Submit: Heather Miller <hcm@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index 729bb8b..3c99457 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -4,9 +4,14 @@
 
 * * *
 
-Milestone 80
+Milestone 81
 
 <Insert new notes here- top is most recent.>
+
+* * *
+
+Milestone 80
+
   * For Vulkan backend, we now require that the VkDevice, Queue, and Instance outlive
     either the destruction or abandoning of the GrContext. Additionally, all
     GrBackendTextures created via GrContext::createBackendTexture calls must be deleted