Update Recommended Java Version

In GCZ Transformer docs, we recommend JDK11 Java Version. However, our pom.xml compiler, the docker images, and OSDU Core Services all use Java 1.8 / JDK8. In addition, we've found that recent versions of JDK11 (11.0.21) have presented error during runtime. For these reasons we have decided to switch our recommended Java version from JDK11 to JDK8. This should have minimal to no impact, given that deployments thus far have leveraged JDK8 or earlier non-problematic versions of JDK11.