Bump VERSION to 4.8.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v4.7.1...10f4fb3d0db2d848edca9184db0dc49b57f30003

• Release notes:

### Doc changes

- !305 Document abbreviation followed by tag

### Other changes (CI, backstage)

- !299 Add release stage to push new gems on tag
- !304 Change the Release issue template to a MR template and improve it
- !306 Show only failed tests by default in the HTML report
- !307 Add slack notification on failure for jobs triggered from gstg-gitlab-qa-orchestrated infra/deployer 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.