Move GitGarbageCollectWorker to Projects namespace

What does this MR do?

This MR is a previous refactor we need for #296892 (closed).

When we implement the issue, we will have several garbage collect workers because each repository and resource behaves differently. In this first step, we move the GitGarbageCollectWorker worker to the Projects namespace because at the moment it only works for projects.

According to our guidelines, we can only remove workers on major releases (https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#removing-workers). Therefore, we need to still maintain the old GitGarbageCollectWorker until %14.0.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Refs #296892 (closed).