chore(opt): add notice about user groups in C8

[PHWaechtler]

related to OPT-7413

Description

User groups are currently not available to assign in Optimize collections for SaaS and CCSM: https://jira.camunda.com/browse/OPT-7413

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.

[christinaausley]

@PHWaechtler -- this should be backported to current and versioned docs, yes?

[PHWaechtler]

@PHWaechtler -- this should be backported to current and versioned docs, yes?

hm tbf we didn't even have collection users management in some modes until recently so I'd say its not crucial but wouldn't hurt. I can have a look later.

[PHWaechtler]

@christinaausley ready for re review :)