Consider not depending on updated_from_web? check for CODEOWNERS

In https://gitlab.com/gitlab-org/gitlab/blob/c89a6b4548e42ed446099c992c5359bfe34d1de2/ee/lib/ee/gitlab/checks/diff_check.rb#L23, we check whether the query parameter protocol is web to determine whether we should validate CODEOWNERS.

I think the goal there is to allow merge requests bypass CODEOWNERS checks, but prevent direct pushes.

If that's the case, we already attempt to detect whether the push is coming from a merge request via MatchingMergeRequest in https://gitlab.com/gitlab-org/gitlab/blob/28439ca4b1dd14f22a5a6ad14530f6bf1046f8bc/lib/gitlab/checks/branch_check.rb#L102. Maybe we should be using that instead of this magic query parameter value?

Or maybe we should be more explicit and instead of the obscure protocol=web field use the query parameter merge_request=true.