Bump VERSION to 6.3.0

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/-/compare/v6.2.1...a29186cd732b9de3b96d4b1db65426a65c60dddd

• Release notes:

### New features and features updates

- !89 Add cop to deprecate `include ActionView::Helpers::UrlHelper`.

• 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.

Related to gitlab-org/gitlab#340567 (closed)