Bump version to 5.1.1

• Diff: v5.1.0...cf63b5ff

• Release notes:

### Other changes (CI, backstage)

- !409 Switch to using incoming webhook for posting to slack.

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Create a tag for the new release version.