Combine Package categories related to package registries

This removes the Maven, npm, RubyGem, Linux, and Conan package repository categories and creates a single category called Package Registry that supports all of these (and more). Content from these other categories was added to the new centralized one.

This will help with the proliferation of categories as we continue to add more and more like Conan and NuGet, and also give us a natural home for the improvements that enhance all of them at once. We were bumping up against the limit as we were adding more, and this change will make it easier to manage the overall strategy related to package management.

TODO after merge:

• need package repository documentation home, currently there is only each package manager doc but no central landing point We have this issue scheduled for 12.2: https://gitlab.com/gitlab-org/gitlab-ce/issues/62600
• remove category maturity labels from old issues
• create new category label
• remove old category labels, add new category label, remove maturity labels from these items
• need new maturity plan for package repository to get it to viable
• create new category epic for package registry, link to it, then delete old one : Addressed in MR !23832 (merged)