Javac source and target compatibility level 8 deprecated in JDK 21

[benknoll-umn]

Releases of OpenJDK will not be able to compile to target Java 8 in the future. Classes compiled to support Java 8 will still run in newer JDKs. However, continued support of Java 8 presents an obstacle to developer usability and especially automated testing as a different JDKs would need to be used to compile and test. Consider the following options:

1. Remove Java 8 support now.
2. Prepare to remove Java 8 support by testing the new -source and -target level. Remove Java 8 support once an LTS version of Java is released that no longer supports targeting Java 8. Document that non-LTS releases not supporting Java 8 are not tested.
3. Maintain the Java 8 target level until sometime into the future, even until Java 8 end-of-life in 2030. Document that all releases not supporting Java 8 are not tested.
4. Update the testing toolchain to compile using one version of the JDK and test using another. Since the integration tests are Python-based there is a possibility of only running the integration tests using the latest JDK, via GitHub action workflows.

If we choose a new -source and -target level we should bump to 11, the next lowest LTS release of Java.

[benknoll-umn]

Decided to remove Java 8 support and move to Java 11+