In https://gitlab.com/gitlab-org/gitlab/-/issues/220771, the
`/api/v4/internal/allowed` endpoint would timeout after 60 seconds due
to this query in `MatchingMergeRequest#match?`.

This commit adds a partial index for locked merge requests to improve
the performance of this query.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/220771