feat: add NeoForge 1.21.1 support

Port WorldHandler to Minecraft 1.21.1 on NeoForge 21.1.225.

This migrates the build from ForgeGradle to NeoForge ModDevGradle, updates mod metadata, replaces Forge API usage with NeoForge equivalents, updates registry access for 1.21.1, and avoids a client-startup advancement reload that can hang large modpacks.

Validated with compileJava and full Gradle build.

gradle.properties

@@ -1,15 +1,16 @@
 # WorldHandler
 mod_id = worldhandler
 mod_name = WorldHandler
-mod_version = 3.5.1
-minecraft_version = 1.20.4
+mod_version = 3.5.2-neoforge
+minecraft_version = 1.21.1
 group = exopandora.worldhandler
 main_class = exopandora.worldhandler.Main
 author = Exopandora
 
-# Forge
-forge_version = 49.0.3
-forge_compatible_minecraft_versions = 1.20.3,1.20.4
+# NeoForge
+neo_version = 21.1.225
+loader_version_range = [4,)
+neoforge_compatible_minecraft_versions = 1.21.1
 
 # Publishing
 curse_project_id = 228970