Update deprecation notice

[saig0]

Description

Update deprecation notice in README. The new testing library will be available for Camunda 8.6 but will not replace ZPT immediately because of missing features.

Related issues

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

 47 files  ±0   47 suites  ±0   1m 10s ⏱️ +6s
135 tests ±0  135 ✅ ±0  0 💤 ±0  0 ❌ ±0 
429 runs  ±0  429 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit e8aaa77. ± Comparison against base commit 805e6f5.

[korthout]

LGTM 👍