Scan for merge commit messages with absolute MR refs

What does this MR do and why?

Currently, QA issues created by the Release Tools Bot do not include merge commits with absolute MR references.

For example, the following substring of a merge commit is matched currently:

See merge request gitlab-org/gitlab!123

However the following absolute reference is not:

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103210

How to verify

Evaluate the following expression on master and this MR's branch:

ReleaseTools::Qa::ProjectChangeset.new(project: ReleaseTools::Project::GitlabEe,
                                       from: 'v15.5.3-ee',
                                       to: 'v15.5.4-ee').merge_requests.pluck("web_url")

Verify that on master you're 3 MRs short:

• gitlab!103643 (merged)
• gitlab!102716 (merged)
• gitlab!103772 (merged)

Author Check-list

• Has documentation been updated?