fix: don't modify ssh_config for ubuntu-cpc projects (LP:1968873)

Modifying directly /etc/ssh/sshd_config creates "problems" when
upgrading eg. from Focal to Jammy because the upgrade will ask the
user what to do with the modified config. To avoid that, put the
custom configuration into /etc/ssh/sshd_config.d/ so the upgrade of
openssh-server can just replace /etc/ssh/sshd_config without asking
the user.

live-build/ubuntu-cpc/hooks.d/chroot/052-ssh_authentication.chroot

@@ -1,3 +1,5 @@
 #!/bin/bash
 
-sed -i "s|#PasswordAuthentication yes|PasswordAuthentication no|g" /etc/ssh/sshd_config
+cat << EOF >> "/etc/ssh/sshd_config.d/10-cloudimg-settings.conf"
+PasswordAuthentication no
+EOF