Switch CI from openjdk to eclipse-temurin images

The openjdk images are no longer updated, aside from early-access
builds. See:

* https://hub.docker.com/_/openjdk/
* https://github.com/docker-library/openjdk/issues/505

.gitlab-ci.yml

-image: openjdk:8-jdk-slim
+image: eclipse-temurin:8
 
 stages:
   - test
@@ -21,11 +21,11 @@ tests:
 
 tests:jdk-11:
   extends: tests
-  image: openjdk:11-jdk-slim
+  image: eclipse-temurin:11
 
 tests:jdk-17:
   extends: tests
-  image: openjdk:17-jdk-slim
+  image: eclipse-temurin:17
 
 publish:
   stage: deploy