Restrict access to release branch: release-R121 to LTS Refresher group. BUG=b/475291916 Change-Id: Ib9de31ac6a15148a40f12e46b1171bc9745fd073 Reviewed-on: https://cos-review.googlesource.com/c/All-Projects/+/129701 Tested-by: Dom Huh <domhuh@google.com> Reviewed-by: Kevin Liu <zhihuil@google.com>