Bump version to 5.2.0

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/compare/v5.1.0...8331cd01b5cf5e29c86accb45951ddd7fc0078f6

• Release notes:

### New features and features updates

- !59 Add cop to detect invalid beginning description characters.

• Checklist before merging:

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

• Checklist after merging:

  • Update the release notes for the newly created tag.