chore(docs): mark assignee/candidate group data reports as C7 only

[RomanJRW]

relates to OPT-7028

Description

Some report types aren't available in C8, but this is not necessarily clear to users. We should make this clearer in our docs.

I know we typically use the label for our docs but that doesn't neatly fit this page, as only a small part is C7-specific. Open to alternatives here @christinaausley :)

When should this change go live?

• This change is not yet live and should not be merged until {ADD_DATE} (apply hold label or convert to draft PR)?
• There is no urgency with this change.
• This change or page is part of a marketing blog, conference talk, or something else on a schedule.
• This functionality is already available but undocumented.
• This is a bug fix or security concern.

PR Checklist

• I have added changes to the relevant /versioned_docs directory, or they are not for an already released version.
• I have added changes to the main /docs directory (aka /next/), or they are not for future versions.
• My changes require an Engineering review, and I've assigned an engineering manager or tech lead as a reviewer, or my changes do not require an Engineering review.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer, or my changes do not require a technical writer review.

[akeller]

Christina is on vacation, so you get me 😄

An admonition is a great way to handle this. My only request is spell C7 all the way out - Camunda Platform 7 - so the brand police don't come for us.