Bump version to 6.5.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.4.1...e90067552506d6222373d50f8d4d06afdc95f30a

• Release notes:

### New scenarios and scenario updates

- !536 Update the Praefect scenario.

### Other changes (CI, backstage)

- !545 Update backstage label to feature::maintenance.

• 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.