Use maven.compiler.release to build for java 8 from jdk 9+

[relud]

per https://www.baeldung.com/maven-java-version#java9

To cross-compile correctly, the new -release option replaces three flags: -source, -target and -bootclasspath.

Note that maven.compiler.release is only valid for jdk 9+, and when set causes maven-compiler-plugin to ignore source and target.

[jklukas]

LGTM

Am I understanding correctly that we'll still be building our production templates with JDK 8, and this change is at this point simply a convenience for development so that you can run tests, etc. locally without having to make sure you're on JDK 8?

[relud]

Am I understanding correctly that we'll still be building our production templates with JDK 8, and this change is at this point simply a convenience for development so that you can run tests, etc. locally without having to make sure you're on JDK 8?

exactly