Expose `squash_on_merge` in REST API

Problem to solve

Exposes a new field squash_on_merge to Merge Request Rest API.

We should also updated the documentation to clarify the distinction between squash and squash_on_merge fields.

Proposal

There is an existing MR that started this discussion: !90893 (closed)

However, we discovered a performance issue (N+1) described in this note.

Testing

Ad test to update squash_on_merge field. Similar to https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/requests/api/merge_requests_spec.rb#L2847

Previous test may be relevant: !90893 (diffs)