Move snippet repository storage classes to namespace

What does this MR do?

According to our guidelines and cops, all classes should be namespaced. Related to the repository storage update, we introduced the new classes for groups in a namespace, but there are others, for projects and snippets, that still follow the old naming. So now, we have mixed namespaces in this functionality.

With this MR we start cleaning that up, and move everything to a namespace.

Since some of the classes are workers or called inside existing workers, we cannot get rid entirely of these classes, and we need to wait for 14.0 to do that.

In specific we do:

• Move SnippetRepositoryStorageMoves entity to Snippets::RepositoryStorageMoves
• Copy SnippetRepositoryStorageMoves model to SnippetsRepositoryStorageMoves
  • Remove all the logic from SnippetRepositoryStorageMoves
  • Make SnippetRepositoryStorageMoves a subclass of Snippets::RepositoryStorageMoves
• Copy SnippetUpdateRepositoryStorageWorker to Snippets::UpdateRepositoryStorageWorker
  • Remove all the logic from SnippetUpdateRepositoryStorageWorker
  • Make SnippetUpdateRepositoryStorageWorker a subclass of Snippets::UpdateRepositoryStorageWorker
• Copy SnippetScheduleBulkRepositoryShardMovesWorker to Snippets::ScheduleBulkRepositoryShardMovesWorker
  • Remove all the logic from SnippetScheduleBulkRepositoryShardMovesWorker
  • Make SnippetScheduleBulkRepositoryShardMovesWorker a subclass of Snippets::ScheduleBulkRepositoryShardMovesWorker
• Replace all references to the old classes with the new ones.

Does this MR meet the acceptance criteria?

Conformity

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

Related to #299853 (closed)