Revert "Restrict access to release branch: release-R121 to LTS Refresher group."

This reverts commit 0c19841a97d43b63066737144202e5f08ae18f42.

Reason for revert: 121 LTS refresh completed.

Change-Id: I63ae49f711fb4134149a5f79c358d1274069a3fd
Reviewed-on: https://cos-review.googlesource.com/c/All-Projects/+/131523
Tested-by: Dom Huh <domhuh@google.com>
Reviewed-by: Kevin Liu <zhihuil@google.com>