Resolve "Deprecate Gemnasium project service"

What does this MR do?

Deprecate the Gemnasium project service.

This will be followed by another maintenance task in the next iteration to totally remove the code related to this service.

Are there points in the code the reviewer needs to double check?

The existing services in DB are being removed using a post_deployment_migrations.

We could probably just flag the existing registered services as active=false as this would be enough to avoid adding jobs to the queue but this also means we'll need another migration in next iteration to remove them. I think we can do both right now but please check this is ok.

Why was this MR needed?

Gemnasium service must be dropped due to the shutdown of gemnasium.com.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46354 (closed)