Unique environment scope for clusters belonging to a group

What does this MR do?

Given we now have group clusters in CE, make sure environment scopes (EE can have different scopes) are not duplicated.

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

Changes to CE are in this CE MR

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/34758

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.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?