Bump version to 5.6.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.6.0...fb3bc97619d5821462f1339c8ffe2fafeb470f31

• Release notes:

### New scenarios and scenario updates

- !437 Add a DB to Test::Integration::Praefect

Although !437 (merged) changes the configuration of the Praefect scenario, it doesn't change how gitlab-qa is used nor how it operates (from the user's perspective), except to be a little more consistent with our recommendations, so I consider it a patch, not a minor feature change.

• 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.