d/openssh-server.{postinst,config}: get_config_option: Replace perl with sed

openssh-server.postinst and openssh-server.config use Perl to get the value of options in /etc/ssh/sshd_config. Such simple processing can be achieved also with a very similar sed invocation.

Using sed instead of Perl reduces the number of packages required to install openssh-server. (Perl is a candidate for removal from essential, see https://salsa.debian.org/gioele/debian-vm-minimal-pkgs and https://wiki.debian.org/Proposals/EssentialOnDiet.)