Enable PROXY protocol for gitlab-sshd

PROXY protocol allows us to track client IP addresses. It'll make IP restriction and rate-limiting by IP address possible.

It has been implemented for gitlab-sshd:

• #491 (closed)
• #532 (closed)

After gitlab-sshd is deployed and stable, we can enable PROXY protocol for users: https://gitlab.com/gitlab-org/gitlab-shell/-/blob/c41d2b585dfec86851821155bef33de9f8783f59/config.yml.example#L76

Documentation

• Update the reference to IP Allowlisting