Bump version to 5.3.0

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/compare/v5.2.0...755a19fe0dbdd9a39f315a4089abfcdabe87abf7

• Release notes:

### New features and features updates

- !63 Use EnforcedStyle: always for Style/AndOr

### Other changes (tooling, technical debt)

- !61 ci: Update default image to 'ruby:2.7'
- !62 Auto-require all cops by default

• 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.