Add JDK 21 to androidx checkout

This is not yet used, but we need it to unblock all the issues with using it

Change-Id: Ia534101b22ee6ea242e6c4dbae28780eb74b7a81
Test: None
diff --git a/default.xml b/default.xml
index 18ee2f5..56c2a5c 100644
--- a/default.xml
+++ b/default.xml
@@ -164,6 +164,11 @@
     name="platform/prebuilts/jdk/jdk17"
     revision="main"
     clone-depth="1" />
+  <project
+    path="prebuilts/jdk/jdk21"
+    name="platform/prebuilts/jdk/jdk21"
+    revision="main"
+    clone-depth="1" />
   
   <!-- Golden images -->
   <project path="golden" name="platform/frameworks/support-golden" clone-depth="1" />