Move Housekeeping service to a different namespace

What does this MR do?

Now that we extracted the model housekeeping logic in !51564 (merged), we need to make the HousekeepingService generic. At the moment, it's stored inside the Projects namespace, but we can move it outside of it, make it generic, and reuse it later for project and group wikis.

Nevertheless, we cannot completely remove the Projects::Housekeeping class because it might be called in sidekiq jobs during the deployment. We need to remove this class in the next milestone after this MR is merged.

Does this MR meet the acceptance criteria?

Conformity

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

Related to #296892 (closed)