Fix a bug where it would try to register legacy channels(!) on 1.13+

proxy/src/main/java/com/velocitypowered/proxy/util/VelocityChannelRegistrar.java

@@ -46,8 +46,10 @@ public class VelocityChannelRegistrar implements ChannelRegistrar {
     public Collection<String> getModernChannelIds() {
         Collection<String> ids = new ArrayList<>();
         for (ChannelIdentifier value : identifierMap.values()) {
+            if (value instanceof MinecraftChannelIdentifier) {
                 ids.add(value.getId());
             }
+        }
         return ids;
     }