Issue using classifier with gradle version 8+

[Symeon94]

Hello,

I'm trying to build with the latest Gradle (it would be handy to have somewhere in the README a mention of the version used to build the library) and I get an error in building

* What went wrong:
A problem occurred evaluating project ':richtextfx'.
> Could not set unknown property 'classifier' for task ':richtextfx:javadocJar' of type org.gradle.api.tasks.bundling.Jar.

From quickly looking, it seems that it is simply due to deprecated "classifier" used in the build: It seems that instead

jar {
    classifier = 'sources'
}

For version 8+ you should use:

jar {
    archiveClassifier.set('sources')
}

[Symeon94]

It seems additionally that the plugin org.unbroken-dome.test-sets version 4.0.0 is not compatible with the 8+ version (for the same reason). Upgrade to version 4.1.0 (the latest fixes it).

We should probably document version in the README to make sure future people wanting to build have the information.

[Jugen]

Have upgraded to Gradle 8.5 and bumped "org.unbroken-dome.test-sets" to version 4.1.0