Resolve "Project Namespace field should be removed from Group Cluster, Add existing"

What does this MR do?

Fixes a ~bug where the Project Namespace field is shown for when adding an existing Group cluster.

What are the relevant issue numbers?

Closes #55090 (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
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer