project.config: lock down release-branch.go1.27

Lock down the release branch to behave like the other release
branches, as per the release process. This effectively undoes
the short-term additional permission granted to all approvers
in CL 799482.

For go.dev/issue/79581.

Change-Id: Icb5ecb0de40f6ee518aef6a29c39cc5de6ecc3dc
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/811600
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>