Update sshd config for Focal

[emkll]

Description

In the current sshd configuration, we specify a path to a host dsa key for the server, but that has been deprecated/disabled. This was uncovered by an ossec alert while reviewing #5638

The alert is as follows:

Dec 1 15:08:22 app-staging sshd[3302]: error: Unable to load host key: /etc/ssh/ssh_host_dsa_key

The server sshd config is

securedrop/install_files/ansible-base/roles/restrict-direct-access/templates/sshd_config

Line 5 in 1d50b6e

HostKey /etc/ssh/ssh_host_dsa_key

and removing the line should suffice. We should also review and audit the current configuration to ensure we adhere to current best practices.