Add group setting to allow/disallow RubyGems duplicates

Proposal

#276882 (closed) added a setting to allow and disallow duplicates for Maven packages. The same should be done for RubyGems.

By default, the GitLab RubyGems registry allows duplicates.

This would be set up in the same way as the Maven settings, adding two new settings to namespace_package_settings:

• A boolean setting rubygems_duplicates_allowed with default: true
• A text setting rubygems_duplicate_exception_regex with default ''.

Updates will need to be made to the GraphQL API and the front end will need to be updated to display the new settings. See #276882 (closed) for the design.