Update dependencies and improve various functionalities

- Bump Gradle to 8.5 and adjust related configs.
- Upgrade from JDK 17 to JDK 21.
- Replace deprecated methods with modern equivalents (`withDisplayName` -> `withCustomName`).
- Refactor and simplify event handling (e.g., PlayerLoginEvent -> AsyncPlayerConfigurationEvent).
- Fix minor issues with player and inventory handling.
- Improve Gradle startup scripts for better compatibility.
- Transition plugin handling from deprecated to current class references.
- General cleanup and code modernization.

build.gradle

@@ -12,7 +12,7 @@ repositories {
 }
 
 dependencies {
-    implementation 'com.github.waxeria:Minestom:e0427a36f3'
+    implementation 'net.minestom:minestom-snapshots:fd51c8d17a'
     implementation 'org.jctools:jctools-core:4.0.1'
     implementation 'com.google.guava:guava:31.0.1-jre'
 }