Bump version to 6.16.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.15.4...2e1f7509000edb9cacaf9dde94fa71a0e2b417d8

• Release notes:

Other changes (CI, backstage)

• !596 (merged) Allow custom default branch.

• 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.