Ensure that buster-backports will be absent on Debian 11+

tasks/setup-debian-vanilla.yml

@@ -2,9 +2,8 @@
 - name: (Debian) Add WireGuard repository on buster
   apt_repository:
     repo: "deb http://deb.debian.org/debian buster-backports main"
-    state: present
+    state: "{{ 'present' if (ansible_distribution_version | int <= 10) else 'absent' }}"
     update_cache: yes
-  when: ansible_distribution_version | int <= 10
   tags:
     - wg-install