This prevents accidental use for release branch merging, which
works but shouldn't.

Change-Id: I4db99bb721c935a8a1c7c44c1b4d909f44a8bf9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3705382
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81157}