Sync `groups` and `group` to fix EE validation

What does this MR do?

Change Clusters::Cluster#group so that EE validation will work in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8124

We populate clusters via the groups association. But we validate via cluster.group in EE. Fix so that cluster.group uses groups association. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8124#note_113063308

What are the relevant issue numbers?

#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
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.