Mac OS Monterey uses port 5000 for Airplay

[robertdstein]

Skyportal uses port 5000 by default, but on the new Mac OS Monterey, this port is by default used for Airplay.

Some background: https://anandtripathi5.medium.com/port-5000-already-in-use-macos-monterey-issue-d86b02edd36c

You can manually disable Airplay receiver, in which case the port is freed up, but a better solution would be to use a different port. 5700 was a popular choice.

(Same issue as skyportal/skyportal#2657)

[stefanv]

Easiest way to address this for now is to document clearly how to change the port number in the config.