fix(subgroups): add verification of group creation capability to subgroup UI

What does this MR do?

This MR hides the "New Subgroup" button in case a user is not allowed to create groups and therefore subgroups in general.

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

Branching: Since this is a minor fix for a regression issue introduced in 9.0, I've created this MR on top of 9-0-stable which is also the target for this MR. I'm unsure if this is the intended branching concept your using.

Why was this MR needed?

User which are not allowed to create groups still saw the "New Subgroup" button. Clicking this button lead to 404 error. This is a simple fix to prevent this faulty behavior.

Screenshots (if relevant)

See issue #30139 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary (fixes a regression introduced in 9.0)
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes issue #30139 (closed)