a8n/core: Support updating changesets on BitbucketServer

Created by: mrnugget

This is a follow-up to #7683 (closed)

Right now we only support updating changesets on GitHub. But since Bitbucket Server is otherwise fully-supported, we need to add this too.

What we need to do:

1. Add a UpdatePullRequest method to the BitbucketServer client.
2. Add UpdateChangeset method to BitbucketServerSource
3. Merge the UpdateChangesetSource back into the ChangesetSource here now that we have feature-parity