Update sshd_config

[paulcalabro]

This disables session multiplexing to fix #42.

More information regarding MaxSessions:

MaxSessions Specifies the maximum number of open shell, login or subsystem (e.g. sftp) sessions permitted per network connection. Multiple sessions may be established by clients that support connection multiplexing. Setting MaxSessions to 1 will effectively disable session multiplexing, whereas setting it to 0 will prevent all shell, login and subsystem sessions while still permitting forwarding. The default is 10.

what

• Disables session multiplexing.

why

• To ensure 2FA is always used.

references

• https://github.com/cloudposse/bastion/issues/42
• https://man.openbsd.org/sshd_config