Bump version to 5.9.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.8.1...d6b6a0a717cbde324ba02786bb5f0788fb01fa5e

• Release notes:

### Testcase Issues Reporter updates

- !426 Improve testcase issue titles

### Fixes

- !427 Disable Metrics/AbcSize cop

• 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.