Hameln

Update Gradle wrapper to 9.1.0
Updated Gradle wrapper properties, binary, and scripts to version 9.1.0 for consistency, fixes, and new features.

Migrate build to Kotlin DSL and centralise dependency versions
Replaced Groovy build.gradle with Kotlin DSL and introduced a version catalogue.
Centralised dependency and plugin versions, improved build performance with caching and parallelism, and modernised shadowJar and runPaper configuration.
Simplified repositories and kept Java 21 as the toolchain.

Bump Minecraft version to 1.21.9
Updated the Minecraft version in the version catalogue to 1.21.9 to align with the latest upstream release and maintain compatibility.