Use fully-qualified module names for builtin Ansible modules

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

roles/matrix-bot-mjolnir/tasks/validate_config.yml

@@ -1,7 +1,7 @@
 ---
 
 - name: Fail if required variables are undefined
-  fail:
+  ansible.builtin.fail:
     msg: "The `{{ item }}` variable must be defined and have a non-null value."
   with_items:
     - "matrix_bot_mjolnir_access_token"