[borgbackup-server] Use restrict directive for authorized_key

As documented in https://borgbackup.readthedocs.io/en/stable/usage/serve.html, openssh-server > 7.2 is available in debian since at least stretch (7.4) so no risk of compatibility issues.

roles/borgbackup-server/templates/authorized_keys.j2

 {{ ansible_header | comment }}
 
-command="borg serve --restrict-to-path {{ borg.path }}",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding {{ vault.borgbackup_ssh_pubkey }}
+command="borg serve --restrict-to-path {{ borg.path }}",restrict {{ vault.borgbackup_ssh_pubkey }}