Bump version to 6.23.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.22.1...b2e312df6d6da831764564eaad1df1700de299d2

• 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.