ubuntu.sh: Upgrade to openjdk 21

Change-Id: Ibcf930e87c75812f53ed3121fcfd3ce94a9418bb
Signed-off-by: Arthur She <arthur.she@linaro.org>
diff --git a/ubuntu.sh b/ubuntu.sh
index 9d3909c..2d577eb 100755
--- a/ubuntu.sh
+++ b/ubuntu.sh
@@ -8,7 +8,7 @@
     curl \
     gnupg \
     gnupg-agent \
-    openjdk-11-jdk-headless \
+    openjdk-21-jdk-headless \
     python3 \
     python3-venv \
     software-properties-common \