server: Remove UPnP support.

[davecgh]

This removes all support for automatic port mapping via UPnP. A warning will now be shown if the CLI flag is specified that it is no longer supported.

The primary motivations for removing it are:

• Automatic port mapping via UPnP has always been controversial due to its security implications
• It has always been disabled by default due to the aforementioned security implications and thus very few people ever used it
• UPnP is increasingly less relevant than it was over a decade ago and has very low real world utility today
• The existing support only ever worked on a limited amount of devices as is typical of UPnP in general due to most routers not implementing the spec properly
• Any perceived convenience realistically no longer applies today because, due to many security issues over the years with the protocol, including some as recent as 2026, nearly all modern routers come with it disabled by default
  • If a user is capable of enabling UPnP on the router, they are equally capable of just mapping the port themselves
• Manual port forwarding is straightforward and far more secure
• IPv6 is increasingly more common where UPnP is irrelevant

Given all of the aforementioned, it really isn't worth the maintenance burden anymore.

[jrick]

won't miss it