feat: bump java default targets to 21

The engine defaulted to Java 11, so any project that did not set the
compatibility preferences emitted class-file major 55 while building
against AGP 9.2.1 and Kotlin 2.2.20. MABS 13 ships JDK 21 and no AGP
below 9, so 11 is no longer a default worth carrying.

Projects that do not set AndroidJavaSourceCompatibility or
AndroidJavaTargetCompatibility now compile at Java 21 and need a JDK 21
or newer build host. MABS 13 agents and CI already provide one, so the
floor is documented rather than newly imposed.

KOTLIN_JVM_TARGET stays null deliberately. Nothing has consumed it since
the KGP wiring was removed in 8d3bd1e, and AGP 9's built-in Kotlin
derives its jvmTarget from the module's Java target compatibility, so
this raise carries Kotlin with it. Pinning it to a literal 21 would
reintroduce a value that can drift from JAVA_TARGET_COMPATIBILITY once
either is overridden, for no gain.

References https://outsystemsrd.atlassian.net/browse/RDMR-1428

Author: EiyuuZack

framework/cdv-gradle-config-defaults.json

@@ -13,7 +13,7 @@
     "IS_GRADLE_PLUGIN_GOOGLE_SERVICES_ENABLED": false,
     "IS_GRADLE_PLUGIN_KOTLIN_ENABLED": false,
     "PACKAGE_NAMESPACE": "org.apache.cordova.hellocordova",
-    "JAVA_SOURCE_COMPATIBILITY": 11,
-    "JAVA_TARGET_COMPATIBILITY": 11,
+    "JAVA_SOURCE_COMPATIBILITY": 21,
+    "JAVA_TARGET_COMPATIBILITY": 21,
     "KOTLIN_JVM_TARGET": null
 }