Ansible: Set transfer method to scp explicitly

This fixes the warnings for the sftp transfer mechanism failing, when sftp is disabled on all hosts using the ssh hardening role.

This should usually be covered with the scp_if_ssh option but it seems like this option is ignored on newer Ansible versions. Which sadly was not mentioned in any changelog :(

ansible.cfg

@@ -6,6 +6,7 @@ vault_password_file = .vault_pass
 
 [ssh_connection]
 scp_if_ssh = true
+transfer_method = scp
 
 [privilege_escalation]
 become_ask_pass = false