Add notice in the UI that reassignment is restricted to enterprise users
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The following discussion from !187395 should be addressed:
- [ ] @justin_ho started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/187395#note_2481730762):
> Thought (non-blocking): We don't really have any indication in the UI when this is `true` (apart from the different selection of users). I think we should create a follow-up to add a small notice to users along the lines of "Reassignment on this group is currently limited to enterprise users" so it's a bit more obvious.
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