change to native ansible iptables flush

roles/raspbian/tasks/main.yml

@@ -17,11 +17,8 @@
   register: boot_cmdline
 
 - name: Flush iptables before changing iptables-legacy
-  shell:
-    cmd: 'iptables -F'
-  register: flush_iptables
-  changed_when:
-    - flush_iptables.rc == 0 and flush_iptables.stdout_lines.count == 0
+  iptables:
+    flush: true
   when:
     - ansible_facts.distribution_release is search("buster")