Bump VERSION to 5.0.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v4.20.0...26c41e8442ffd85e875af386dc9138dd595b7ac8

• Release notes:

### Breaking changes

- !397 Add `--report-in-issues` to use issues to track tests.
  - This includes a new `gitlab-qa-report` exe. The `--prepare-stage-reports` feature is now accessible via `gitlab-qa-report` instead of `gitlab-qa`

### Other changes (CI, backstage)

- !390 Don't run pipelines for tags
- !391 Remove ce:custom-parallel jobs
- !392 Remove CE quarantine and Staging jobs
- !394 Add CE and Staging jobs back and exclude jobs that don't match RELEASE
- !395 Re-enable git protocol v2 testing
- !396 Exclude .only-qa jobs from all triggered pipelines

• 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.