Polish JDK 18 CI pipeline

This commit ensures that JDK 18 is properly installed in the CI image.

See gh-27607

ci/images/setup.sh

@@ -22,7 +22,7 @@ curl --output /opt/concourse-release-scripts.jar https://repo.spring.io/release/
 
 mkdir -p /opt/openjdk
 pushd /opt/openjdk > /dev/null
-for jdk in java17
+for jdk in java17 java18
 do
   JDK_URL=$( /get-jdk-url.sh $jdk )
   mkdir $jdk