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.

.idea/misc.xml

@@ -1,6 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="ExternalStorageConfigurationManager" enabled="true" />
-  <component name="ProjectRootManager" version="2" languageLevel="JDK_17" default="true" project-jdk-name="corretto-17" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_21" project-jdk-name="corretto-17" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
   </component>
 </project>