Upgrade Mixin target version to 21

Author: Avanatiker

src/main/resources/lambda.mixins.common.json

@@ -2,7 +2,7 @@
   "required": true,
   "minVersion": "0.8",
   "package": "com.lambda.mixin",
-  "compatibilityLevel": "JAVA_17",
+  "compatibilityLevel": "JAVA_21",
   "client": [
     "CrashReportMixin",
     "MinecraftClientMixin",