Move gradle setup to kotlin dsl, add paperweight 1.21.3 module

[Joo200]

This PR includes:

• Moved build files from groovy to kotlin dsl. Gradle can handle mixed layouts so not every directory is converted to kotlin dsl. I can move the remaining stuff to but I want to be sure that the changes are working correctly first.
• The change to kotlin dsl was the preparation to use paperweight instead of spigot:
  • Added paperweight module for bukkit 1.21.3
• Validated plugin build process and functions on spigot and paper 1.21.3

Some more ToDos:

• What should happen with other build.gradle files? convert them all?
• Improve the build logic further. E.g. with shared build-logic