Bump VERSION to 4.8.1

This release is merely to make sure that the release job now works as expected (there's no new feature or bug fix per se).

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v4.8.0...56c7942

• Release notes:

### Doc changes

- !310 Update release process regarding opening an MR.

### Other changes (CI, backstage)

- !312 Fix the `release` job and make it manual for master + non-schedule/tag.
- !313 Update the Ruby and gem version used in the `release` job.
- !316 Fix the `release` job by using the 'gem' commands directly.
- !318 Fix the `only:variables` condition for the `notify_slack` job.

• 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.

/label Quality ~backstage