CI: Downgrade compile JDK to 21

Change-Id: I2df97646768d0cfcd70a8169737daf0f4652e015
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index c29556e..4b6ee3c 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-image: eclipse-temurin:25
+image: eclipse-temurin:21
 
 stages:
     - build