Bump version to 8.12.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.12.0...0800e9c9c07cc56555b0700851cc2456fc615efe

• Release notes:

### Fixes

- !1085 Drop v prefix from QA image tag

### Doc changes

- !1086 Add documentation for Test::Omnibus::UpdateFromPrevious scenario

### Other changes (CI, backstage)

- !1083 Add FIPS environment variable

• 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.