The Bitbucket pull request API
(https://developer.atlassian.com/bitbucket/api/2/reference/) reports
four different states for pull requests:

1. MERGED
2. SUPERSEDED
3. OPEN
4. DECLINED

Previously, we were not handling the SUPERSEDED case. This commit now
changes SUPERSEDED pull requests as closed.

Possible fix for https://gitlab.com/gitlab-org/gitlab/issues/34656