CI: Use default-jdk-headless for builds

Makes it unnecessary to create they symbolic link to /usr/lib/jvm/default-java ourselves

.gitlab-ci.yml

@@ -79,8 +79,7 @@ build_docker:
         - apt-get -qqqy install debhelper ant debconf gettext libgmp-dev po-debconf fakeroot
             build-essential quilt dh-apparmor libservice-wrapper-java libjson-simple-java
             devscripts libjetty9-java libtomcat9-java libtaglibs-standard-jstlel-java libgetopt-java
-            openjdk-11-jdk-headless bash-completion
-        - ln -s /usr/lib/jvm/java-11-openjdk* /usr/lib/jvm/default-java
+            default-jdk-headless bash-completion
     script:
         - ant debian
     only:
@@ -103,5 +102,4 @@ build_ubuntu-bionic:
         - apt-get -qqqy install debhelper ant debconf gettext libgmp-dev po-debconf fakeroot
             build-essential quilt dh-apparmor libservice-wrapper-java libjson-simple-java
             devscripts libjetty9-java libtomcat9-java libtaglibs-standard-jstlel-java libgetopt-java
-            bash-completion openjdk-8-jdk-headless
-        - ln -nfs /usr/lib/jvm/java-8-openjdk* /usr/lib/jvm/default-java
+            bash-completion default-jdk-headless