Deregister Suggested Reviewers when related project setting is disabled

🧩 Problem to solve

The users are unable to deregister from Suggested Reviewers. The project setting just disables the state in the GitLab database but there is no communication sent back to Suggested Reviewers services. As a result, downstream processes, e.g. data extraction and feature set building, do not know about the deregistration and fail.

💡 Proposal

• Wire the existing GRPC Deregister call to the GitLab Suggested Reviewers client
• Put it behind a background job
• Add appropriate feature_category and monitoring