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>
2 files changed