Bump version to 6.23.0

• Diff: v6.22.1...b2e312df

• Release notes:

### Other changes (CI, backstage)

- !665 Introduce a new `--qa-image QA_IMAGE` option allowing to use custom QA image
- !663 Remove --clean from bundle install on our pipeline

• 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:

  • Update the release notes for the newly created tag.