Added Velocity Modern Forwarding Support

As the title says. Check https://github.com/sekwah41/Advanced-Portals/blob/release/spigot-1.13-1.16/docs/CHANGELOG.md for full changelogs.

Just a notice we likely won't be keeping the released up to date unless I automate it. So check dev.bukkit.org/bukkit-plugins/advanced-portals/ for all available versions and the above changelogs to see what features are added in each version.