[Index state tracking: Deletion] ProcessPendingDeletionRepositoriesWorker: Handle project deletion

Implement green tasks

Find projects in pending_deletion state and handle the deletion:

1. Call the indexer with delete operation to delete all documents linked to project
2. Mark repository as deleted and set deletion reason

JBTD:

• A worker added to SchedulingWorker that processes pending deletion repositories

Suggested worker name: ProcessPendingDeletionRepositoriesWorker