Not able to build on jdk 20

[stevenschlansker]

I am running into a similar, or the same, issue as #268 - internal compiler errors in Eclipse.
I am trying to add additional debugging, to better understand the cause of the failure. However, FreeBuilder does not seem to build on JDK20.

When using the included gradle wrapper,

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring root project 'FreeBuilder'.
> Could not open cp_proj generic class cache for build file '/Users/steven/code/FreeBuilder/build.gradle' (/Users/steven/.gradle/caches/6.8.3/scripts/dagdulbidv8m96upkkank21ny).
   > BUG! exception in phase 'semantic analysis' in source unit '_BuildScript_' Unsupported class file major version 64

When I tried to install Gradle 8.2 and run that,

FAILURE: Build failed with an exception.

* Where:
Build file '/Users/steven/code/FreeBuilder/build.gradle' line: 17

* What went wrong:
An exception occurred applying plugin request [id: 'com.github.johnrengelman.shadow', version: '4.0.3']
> Failed to apply plugin class 'com.github.jengelman.gradle.plugins.shadow.ShadowJavaPlugin'.
   > You can't map a property that does not exist: propertyName=classifier

It would be nice if the FreeBuilder build process supported using JDK 20, even if the project of course still targets older releases.