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The openjdk images are no longer updated, aside from early-access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
   - deploy