Fix new branch button spec
- We can create the note directly on the issue, rather than attaching it after creation.
- We need to update the MergeRequestClosesIssues relation to ensure that the issue know that it's closed by the MR.
- We should also check that the unavailable button is displayed - not just that the available button is displayed.
Honestly, I'm not sure how this spec was passing on CI most of the time
Closes #24089 (closed).