Ensure runner_type is updated correctly when assigning shared runner to project

What does this MR do?

At the moment we are transition from is_shared boolean to runner_type enum and they are supposed to remain in sync until we deprecate is_shared boolean. This piece of logic was missed in !9646 (merged)

Additionally we want to prevent transitions the new group runners to project runners as this requires more consideration to get right and it would have led to inconsistent states.

This assign_to logic is executed in both the API and the runner admin pages, however the API already prevents transitioning shared or group runners so this change will only affect the admin runner pages.

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

Why was this MR needed?

Screenshots (if relevant)

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
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?