Project::ClustersController spec improvement

What does this MR do?

Move go method to the top of the context block

Re-organize 'PUT update' describe block to remove un-necessary context nesting.

DRY up repeated blocks to add_maintainer and sign_in

What are the relevant issue numbers?

Helps with #34758 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides