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.

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,7 @@
-#Fri Apr 14 00:18:38 CEST 2023
-distributionUrl=https\://services.gradle.org/distributions/gradle-7.4-bin.zip
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.5-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists