Add RubyGems to the package protection rule form

What does this MR do and why?

Adds RubyGems to the package type dropdown in the package protection rule form. Users can then pick RubyGems when they add a rule in the project settings.

Related to #603267.

• Stacked on !264053 (merged), which adds the RUBYGEMS GraphQL value. Backend and frontend GraphQL changes ship in separate MRs, per https://docs.gitlab.com/development/graphql_guide/reviewing/.
• Change: one option in packages_protection_rule_form.vue and one line in its spec.
• Translation: the PackageRegistry|RubyGems msgid is already in locale/gitlab.pot.

How to set up and validate locally

1. Check out this branch, with the rubygem_packages flag on.
2. Go to a project's Settings > Packages and registries > Package protection rules and select Add protection rule.
3. Open Package type. RubyGems is in the list.

Screenshots or screen recordings

N/A — one new option in an existing dropdown.

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