Bump target JDK to 21

Change-Id: Ibbe0d6450c9d54018d8b883332d7e351f286e603
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 6a77a75..a4dbda2 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-image: eclipse-temurin:19
+image: eclipse-temurin:21
 
 stages:
     - build