Bump version to 5.16.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.15.0...a79e27f26aa34d6ae748b3a7f2660367b9e83207

• Release notes:

### New scenarios and scenario updates

- !491 Update Geo Omnibus config for Puma and Packages feature
- !486 Use recommended Gitaly Cluster config

### Other changes (CI, backstage)

- !490 Pass through non cluster storage var
- !492 Let us set staging job run params

• 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.