Confirm protected branch before running checks

What does this MR do?

In order to allow for a more targeted application of CodeOwner rules, I extracted the logic into Gitlab::CodeOwenrs::Validator (!32225 (merged) ) - great! The existing use case, in ee/lib/ee/gitlab/checks/diff_check.rb contained the checks for whether or not the target branch was a protected branch with codeowner checks enabled. This logic did not get extracted to the validator, so was missing when the validator was applied to the file edit. !33310 (merged) adds this logic to Gitlab::CodeOwenrs::Validator.

Closes #219340 (closed)

Edited by Yorick Peterse