Add Issue related_merge_requests endpoint

[ddavison]

• Add related_merge_request Issue endpoint (https://docs.gitlab.com/ee/api/issues.html#list-merge-requests-related-to-issue)
• Fix outstanding rubocop issue
• Fix Actions; Ruby/Bundle version incompatibilities

$ rake rubocop
Offenses:

lib/gitlab/cli_helpers.rb:23:48: C: [Correctable] Style/RedundantParentheses: Don't use parentheses around a logical expression.
      @client ||= Gitlab::Client.new(endpoint: (Gitlab.endpoint || ''))
                                               ^^^^^^^^^^^^^^^^^^^^^^^

165 files inspected, 1 offense detected, 1 offense autocorrectable
RuboCop failed!

[ddavison]

This is an upstream fix, after this comment was spotted.

[ddavison]

@NARKOZ — This one is ready for review! 🙏

Also, it looks like the Actions was having an issue with the Ruby/Bundle versioning (example of that from #680). This PR should fix that workflow.