Upgrade build to Gradle 9 #351

Gradle9 sees some in-use methods being removed. Moving to Gradle9 build would require either invoking methods via reflect (ugly) or dropping support for some older versions of Gradle, specifically, it looks like the oldest Gradle the plugin would support would change from 5.1 to 7.1.

This would be a breaking change, requiring a major version bump.

It's worth noting that the 2.0.0 release was supposed to move the plugin to requiring Java 17, but that doesn't seem to be the case. Target Java version is still 11. Many libraries are moving to having Java 17 as a minimum and being on Java 11 is already starting to mean some dependency can't, or are a pain, to update.

A 3.0.0 release could move to Java 17 and a Gradle 9 build, with oldest supported Gradle version of 7.1

I'd appreciate thoughts from the community on this.