Evaluate use of PerSource* options for sshd

[sxa]

A series of PerSource directives was introduced in OpenSSH 9.8 for blocking repeated connection attempts. We should evaluate these options and set them up on our machines to attempt to mitigate denial-of-service attempts on our machines where a suitable version is available.

Ref: https://www.openssh.com/txt/release-9.8 and the details in https://man.openbsd.org/sshd_config (That openbsd page is linked from the official openssh site)