Bump version to 5.1.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.0.3...e9022afdc085e18e2f4a33cbb5bb4535d204dc36

• 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.