Backport #38319 by @bircni
The `official` flag of a pull request review is computed against the
target branch's protection rules at submit time and stored on the review
record. `ChangeTargetBranch` updated the base branch but never
re-evaluated it, so an `official` approval obtained against an
unprotected branch could be retargeted onto a protected branch and
satisfy its required approvals, bypassing the branch protection.
This re-evaluates the `official` flag of the latest approve/reject
reviews against the new base branch whenever the target branch changes.
Co-authored-by: bircni <bircni@icloud.com>
74ad781db9