ansible: jenkins: use default-jre-headless

Use default-jre-headless instead of giving the openjdk version, so we
don't need to adjust it to whatever the current version in debian is.

The default-jre-headless package is at least available since debian 10:
https://packages.debian.org/search?suite=buster&arch=any&searchon=names&keywords=default-jre-headless

Related: OS#6186
Change-Id: I6c5c74ff3d75dca2f89765d95c4b4c6d67ace703
diff --git a/ansible/roles/osmocom-jenkins-slave/tasks/debian.yml b/ansible/roles/osmocom-jenkins-slave/tasks/debian.yml
index ead03d7..3743043 100644
--- a/ansible/roles/osmocom-jenkins-slave/tasks/debian.yml
+++ b/ansible/roles/osmocom-jenkins-slave/tasks/debian.yml
@@ -49,7 +49,7 @@
 - name: install java for buster and later
   apt:
     name:
-      - openjdk-11-jre-headless
+      - default-jre-headless
     cache_valid_time: 3600
     update_cache: yes
     install_recommends: no