Allow experimental use of Java 21 by default in makefiles

Bug: 342332820
Test: EXPERIMENTAL_TARGET_JAVA_VERSION_21=true m javac-check lint-check dist
Change-Id: I510746d617c49a0423f1ab0171a3474b181c3875
diff --git a/core/java_common.mk b/core/java_common.mk
index 65feb15..a21f062 100644
--- a/core/java_common.mk
+++ b/core/java_common.mk
@@ -32,6 +32,8 @@
     else ifneq (,$(LOCAL_SDK_VERSION)$(TARGET_BUILD_USE_PREBUILT_SDKS))
       # TODO(ccross): allow 1.9 for current and unbundled once we have SDK system modules
       LOCAL_JAVA_LANGUAGE_VERSION := 1.8
+    else ifeq ($(EXPERIMENTAL_TARGET_JAVA_VERSION_21),true)
+      LOCAL_JAVA_LANGUAGE_VERSION := 21
     else
       LOCAL_JAVA_LANGUAGE_VERSION := 17
     endif