`setMain` in `JRubyExec` is removed in Gradle 8 and hence the plugin doesn't work

[Here](https://github.com/jruby-gradle/jruby-gradle-plugin/blob/024713cc95d84fb823cef02cc48a5b881719048b/base-plugin/src/main/groovy/com/github/jrubygradle/JRubyExec.groovy#L72) and [here](https://github.com/jruby-gradle/jruby-gradle-plugin/blob/024713cc95d84fb823cef02cc48a5b881719048b/base-plugin/src/main/groovy/com/github/jrubygradle/JRubyExec.groovy#L314).

cc @donv, @UweKubosch 