r/ssh-host-certs: Fix restart handler

The _ssh-host-certs.target_ unit does not exist any more.  It was
provided by the _sshca-cli-systemd_ package to allow machines to
automatically request their SSH host certificates on first boot.  It had
a `ConditionFirstBoot=` requirement, which made it not work at any other
time, so there was no reason to move it into the Ansible configuration
policy.  Instead, we can use the _ssh-host-certs-renew.target_ unit to
trigger requesting or renewing host certificates.

roles/ssh-host-certs/handlers/main.yml

@@ -1,6 +1,6 @@
-- name: restart ssh-host-certs.target
+- name: restart ssh-host-certs-renew.target
   systemd:
-    name: ssh-host-certs.target
+    name: ssh-host-certs-renew.target
     state: started
 
 - name: reload sshd