[chore] Address deprecation of JavaExecSpec.main property

Gradle has deprecated the `JavaExecSpec.getMain()` and `JavaExecSpec.setMain()`
methods in favour of the new `mainClass` property.
The old methods are scheduled to be removed in Gradle 8.0, but Gradle already
warns about their deprecation when used.

This change addresses this issue by using the `mainClass` property if it
exists and fallback to the `setMain` when it doesn't to provide
compatibility with older Gradle versions.

src/main/groovy/org/kordamp/gradle/plugin/jandex/tasks/JandexTask.groovy

@@ -123,7 +123,11 @@ class JandexTask extends DefaultTask {
  args << destination.asFile.get().absolutePath
  args.addAll(resolveSources())
 
- jes.main = JandexMain.class.name
+ if (jes.hasProperty("mainClass")) {
+ jes.mainClass = JandexMain.class.name
+ } else {
+ jes.main = JandexMain.class.name
+ }
  jes.classpath(resolveClasspath())
  jes.args(args)
  }