Verify `gitlab-styles` 13.1.0

Following the first few steps of "Verify upcoming release
of gitlab-styles" from [Checklist][1].

Specifically, ran:
```console
$ bundle install && \
  bundle exec bundler-checksum init && \
  bundle install --gemfile Gemfile.next && \
  cp Gemfile.lock Gemfile.next.lock && \
  BUNDLE_GEMFILE=Gemfile.next bundle lock && \
  BUNDLE_GEMFILE=Gemfile.next bundle exec bundler-checksum init && \
  bundle exec rubocop
...
41153 files inspected, 127 offenses detected, 124 offenses autocorrectable
```

[1]: https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/merge_request_templates/New%20Version%20of%20gitlab-styles.md#checklist