.gitlab-ci.yml: revise combinations of JDK and OS

There are now 3 jobs:

* using OpenJDK 8 on Debian Sid,
* using OpenJDK 11 on Debian Bullseye (our current reference), and
* using OpenJDK 17 on Debian Bookworm (our future reference)

.gitlab-ci.yml

@@ -4,16 +4,20 @@ before_script:
 after_script:
   - gradle --version
 
-focal-jdk8:
-  image: ubuntu:focal
+sid-jdk8:
+  image: debian:sid-slim
   script:
-    - apt-get -y install openjdk-8-jdk
-    - apt-get -y install gradle
+    - apt-get -y install openjdk-8-jdk gradle
     - gradle build --stacktrace
 
-focal-jdk11:
-  image: ubuntu:focal
+bullseye-jdk11:
+  image: debian:bullseye-slim
   script:
-    - apt-get -y install openjdk-11-jdk
-    - apt-get -y install gradle
+    - apt-get -y install openjdk-11-jdk gradle
+    - gradle build --stacktrace
+
+bookworm-jdk17:
+  image: debian:bookworm-slim
+  script:
+    - apt-get -y install openjdk-17-jdk gradle
     - gradle build --stacktrace