Add note that Spring support is unavailable in 8.6+

[akeller]

Description

Spring support with Zeebe Process Test is not available from 8.6+ and comes from a community-supported SDK. This is not clear and causing support tickets, so we add a note that this is unavailable.

In addition, I removed the step linking to the fully supported Spring Zeebe SDK. It's not applicable and redundant (you get this info on the ZPT repo README, including the annotation spring-zeebe-test.).

More internal context can be found here.

Related to camunda/zeebe-process-test#1282.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.