Refactor MergeRequest#current_patch_id_sha to only call MergeRequestDiff#patch_id_sha

Background

It was discussed in !131986 (comment 1568895226) that we should make MergeRequestDiff#patch_id_sha as SSOT for getting patch-id of a merge request.

As of this writing, MergeRequestDiff#patch_id_sha isn't fully implemented yet. It also doesn't support backfilling on access as well.

Proposal

Once MergeRequestDiff#patch_id_sha is fully implemented (with backfilling on access logic), we can refactor MergeRequest#current_patch_id_sha to call MergeRequest#patch_id_sha.