Removed broken code
There were originally plans to use the api from mcsrvstat.us to check if port mapping was successful but I have decided to forego this functionality for now. It may or may not be a feature of a future release.
The plugin will now load and should work on older server versions. As a result you may now see the following in your logs:
[12:02:55 WARN]: [org.bukkit.craftbukkit.v1_19_R3.legacy.CraftLegacy] Initializing Legacy Material Support. Unless you have legacy plugins and/or data this is a bug!
[12:02:57 WARN]: Legacy plugin UPnP v1.4-SNAPSHOT does not specify an api-version.
This is a result of allowing the plugin to support multiple versions in one file.