Set source and target compatibility to Java 21

Change-Id: I0f18b6b1cb919c5b5566de5de0a52fbdff69e5eb
diff --git a/app/build.gradle b/app/build.gradle
index 1ae13bb..8ff5c3b 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -65,15 +65,10 @@
     }
 }
 
-// Enforce JDK 21 Toolchain and Bytecode-Level 21
+// Target JVM 21 compatibility using whatever host JDK is available (minimum 21)
 java {
-    toolchain {
-        languageVersion = JavaLanguageVersion.of(21)
-    }
-}
-
-kotlin {
-    jvmToolchain(21)
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
 }
 
 // For any existing Java source code