Fix authorized_key FQCN and add ansible.posix to CI collections

ansible.posix.authorized_key replaces the bare module name removed
in ansible-core 2.x.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ansible/roles/common/tasks/main.yml

@@ -11,7 +11,7 @@
     validate: visudo -cf %s
 
 - name: Ensure admin SSH key is authorized
-  authorized_key:
+  ansible.posix.authorized_key:
     user: "{{ admin_user }}"
     key: "{{ admin_ssh_key }}"
     state: present