Document the required JDK version for KoP

After the internal discussion in StreamNative, we decide to apply the schema registry feature that upgrades the dependency to JDK 11 to branch-2.9.2 and branch-2.10.0 because the sn-pulsar docker images use JDK11. But for open source users, we should highlight the required JDK version.

In addition, we'd better add a workflow to compile KoP with JDK 8 for older branches.