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.

More internal context can be found here.

Related issues

closes #

Definition of Done

Not all items need to be done depending on the issue and the pull request.

Code changes:

• The changes are backwards compatibility with previous versions
• If it fixes a bug then PRs are created to backport the fix

Testing:

• There are unit/integration tests that verify all acceptance criterias of the issue
• New tests are written to ensure backwards compatibility with further versions
• The behavior is tested manually

Documentation:

• Javadoc has been written
• The documentation is updated

[github-actions]

Test Results

0 files   -  49  0 suites   - 49   0s ⏱️ - 1m 33s
0 tests  - 138  0 ✅  - 138  0 💤 ±0  0 ❌ ±0 
0 runs   - 435  0 ✅  - 435  0 💤 ±0  0 ❌ ±0 

Results for commit 7ede3d7. ± Comparison against base commit bdaa836.

[saig0]

camunda/camunda-docs#4764 is merged. ✅