chore(config): remove discontinued secondary STUN servers (fixes #10011) (#10012)

Similarly to #10009, we will remove some discontinued STUN servers,
except instead of being the official primary server, it's some
unofficial secondary STUN servers.

### Testing

Use a STUN client (like [`pystun3`](https://pypi.org/project/pystun3))
to probe that the removed STUN servers are inactive.

### Documentation

syncthing/docs#902

lib/config/config.go

@@ -75,19 +75,15 @@ var (
 		//"stun.syncthing.net:3478",
 	}
 	DefaultSecondaryStunServers = []string{
-		"stun.callwithus.com:3478",
 		"stun.counterpath.com:3478",
 		"stun.counterpath.net:3478",
 		"stun.ekiga.net:3478",
 		"stun.hitv.com:3478",
-		"stun.ideasip.com:3478",
 		"stun.internetcalls.com:3478",
 		"stun.miwifi.com:3478",
 		"stun.schlund.de:3478",
-		"stun.sipgate.net:10000",
 		"stun.sipgate.net:3478",
 		"stun.voip.aebc.com:3478",
-		"stun.voiparound.com:3478",
 		"stun.voipbuster.com:3478",
 		"stun.voipstunt.com:3478",
 		"stun.xten.com:3478",