Introduce merge request regex title

What does this MR do and why?

In !184935 (merged) we created a project setting for setting a required merge request title.

Here we are now creating the mergeability check to ensure that the title matches.

Important to note: It hasn't been fully integrated in the auto-merge feature, this will be done in a follow up issue.

Related to Enforce naming convention on MR title (#25689)

To Test

• Enable feature flag merge_request_title_regex
• Create an MR
• Go to the project settings -> Merge Requests
• Change the title regex to something
• Reload the MR and check that the mergeability check fails