Oh it’s talking about SSH and not HTTPS.
You can customize the sshd config here: https://github.com/roots/trellis/blob/1163c87b5a7512c212146b2edeb9aed895347a7a/roles/sshd/templates/sshd_config.j2
There’s a Protocol setting which can be set to 2. Although the docs say that 2 is the default.
If you figure this out then it would be nice to submit a PR to Trellis itself.