Bump VERSION to 2.3.1

• Diff: https://gitlab.com/gitlab-org/gitlab-dangerfiles/compare/v2.3.0...9e67daee9fe02e84cfbd2bcad37cf13cbeaab0d7

• Release notes:

### Fixes

- !63 Fix CommitLinter wrongly detecting short references in URLs.

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/dangerfiles/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Update the release notes for the newly created tag.