Remove create MR button from issues when MRs are disabled

What does this MR do?

Removes the create MR button from the issue view when merge requests for the project are disabled.

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

Why was this MR needed?

Because when MRs are disabled the button fails to complete the action.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] 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?

Closes #38247 (closed)