Add flag to control if to target java 21 by default

Bug: 342332820
Change-Id: I76a3212ca6ee00ef9f78839d5873b49aff4f75af
Test: m
Test: lunch aosp_cf_x86_64_phone-trunk_staging-eng && \
       build-flag set RELEASE_TARGET_JAVA_21 true && m

flag_declarations/RELEASE_TARGET_JAVA_21.textproto

diff --git a/flag_declarations/RELEASE_TARGET_JAVA_21.textproto b/flag_declarations/RELEASE_TARGET_JAVA_21.textproto
new file mode 100644
index 0000000..acace44
--- /dev/null
+++ b/flag_declarations/RELEASE_TARGET_JAVA_21.textproto
@@ -0,0 +1,11 @@
+name: "RELEASE_TARGET_JAVA_21"
+namespace: "core_libraries"
+description: "Control if we should target java 21."
+value: {
+  bool_value: false
+}
+workflow: LAUNCH
+containers: "product"
+containers: "system"
+containers: "system_ext"
+containers: "vendor"