Bump version to 5.1.0

• Diff: v5.0.3...e9022afd

• Release notes:

### Other changes (CI, backstage)

- !399 Add ability to post stage-wise results 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.