Update README: Reflect Shadow Plugin Changes and Kotlin DSL

[ProfessorSam]

Hey!

While working on relocating FastBoard with Java 21, I encountered an issue where the shadow plugin doesn't support Java 21. During my search for a compatible version, I discovered that the package has been moved from com.github.johnrengelman.shadow to com.gradleup.shadow starting from version 8.3.0. To help others avoid this issue, I thought it would be helpful to update the README to reflect this change. Additionally, since Gradle uses Kotlin DSL as the default (as mentioned in this article), I've also updated the README accordingly.

Thanks for all your hard work! ❤️

Best,
Sam

[MrMicky-FR]

Thank you for this pull request, it's a good idea to update the shadow plugin!

As far as the use of Kotlin is concerned, Groovy is still widely used at the moment (for example, the Shadow plugin documentation and the Gradle documentation use Groovy by default). Also, beginners, who are the ones most likely to use the example in the README, are more likely to use Groovy as most of the resources online are for Groovy, so I think it would be better to delay the migration from Groovy to Kotlin for now