Adds axe feature tests for packages dependency proxy

What does this MR do and why?

We need to test the accessibility for recently added settings page for package dependency proxy

• axe test caught the missing label-for field, fixed frontend
• Need not skip link-in-text-block on package registry settings page anymore, so removed that as well

MR acceptance checklist

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Screenshots or screen recordings

N/A

How to set up and validate locally

bundle exec rspec spec/features/projects/settings/registry_settings_cleanup_tags_spec.rb spec/features/projects/settings/registry_settings_spec.rb ee/spec/features/projects/settings/packages_spec.rb

Related to #410726 (closed)