Test Java 21 and identify next steps after illegal-access workaround

[spbolton]

Specific: Run PR builds after updating configuration to use Java 21 after the changes to testing framework and illegal-access to try to see if we can now run all integration and postman tests to create a clean build/test run on Java 21. This should not be merged into master but just seeing if we are prepared.

Measurable: If Successful create issues for the next steps to move to Java 21 through the build pipeline and any disruption to Customers, If unsuccessful document and create issues that is blocking full build/test on Java 21 so they can be prioritized

Achievable: We are not unrealistically expecting that we can resolve the illegal-access issue, this will get us a step forward. In the process, we may find we are very close and discover the next steps either to move onto full deployment or further remedial work required.

Relevant: Even if we have fixed illegal-access issues with java21, there may be other issues uncovered or this may work differently in Java 21, we need to test to see how close we are after the changes.

Time-bound: Document these changes within 1 sprint

[spbolton]

The epic #27993 was updated with the remaining tasks