fix: insert iptables rules at the top to avoid conflicting with the DENY ALL rule

tasks/configure_iptables_externalusers.yml

 ---
 - name: iptables-Regeln setzen (ZIH der TU Dresden)
   iptables:
-    action: append
+    action: insert
     chain: INPUT
     comment: "{{ item.comment }}"
     destination: "{{ item.dest | default(omit) }}"