gitlab-sshd: MaxStartup limit settings

OpenSSH supports the MaxStartups configuration parameter (https://manpages.ubuntu.com/manpages/kinetic/en/man5/sshd_config.5.html):

MaxStartups
  Specifies the maximum number of concurrent unauthenticated connections to the SSH
  daemon.  Additional connections will be dropped until authentication succeeds or the
  LoginGraceTime expires for a connection.  The default is 10:30:100.

  Alternatively, random early drop can be enabled by specifying the three colon
  separated values start:rate:full (e.g. "10:30:60").  sshd(8) will refuse connection
  attempts with a probability of rate/100 (30%) if there are currently start (10)
  unauthenticated connections.  The probability increases linearly and all connection
  attempts are refused if the number of unauthenticated connections reaches full (60).

We might want to have a similar feature to gitlab-sshd.

References: https://about.gitlab.com/blog/2019/08/27/tyranny-of-the-clock/