Removes `CreateTriggerRequestService` and add a blocker to prevent saving variables on `Ci::TriggerRequest`

What does this MR do?

This MR removes CreateTriggerRequestService and add a blocker to prevent saving variables on Ci::TriggerRequest

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

Doesn't this break a compatibility on out side of GitLab?

Why was this MR needed?

We were supposed to wait until API v3 has completely removed. (Because it's going to be easy to work) Although we decided to work on now for bashing this technical debt.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/35650