use github action for ant build and ensure code works on multiple Java versions

[Samael]

I forked the repo to look into adding support for JPMS but wanted to ensure build pipeline is in place first. So I created a branch that has a workflow which builds using Java 8, 11, 17, and 20. For the older JDK versions it's still possible to target Java 7 but newer versions have to target JDK 8 or above. Note that I had to increase the Java version for source/target from 5 to 7.