Update release notes for M87 branch cut, M88 begin

Bug: skia:
Change-Id: Id4cc2f42389dac1e087026fee5c0df0eff4e89c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/321776
Reviewed-by: Heather Miller <hcm@google.com>
Auto-Submit: Heather Miller <hcm@google.com>
Commit-Queue: Heather Miller <hcm@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index a43aff0..7c728d9 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -4,11 +4,16 @@
 
 * * *
 
-Milestone 87
+Milestone 88
 ------------
 
   * <insert new release notes here>
 
+* * *
+
+Milestone 87
+------------
+
   * GrVkImageInfo now has a field for sample count. GrBackendRenderTarget constructor
     that took both a GrVkImageInfo and separate sample count is deprecated. Use the
     version without sample count instead. Similarly, GrD3DTextureResourceInfo now