Switch to Mojang mappings

ForgeGradle (probably sensibly) yells at me about doing this. However:
 - There's a reasonable number of mods doing this, which establishes
   some optimistic precedent.
 - The licence update in Aug 2020 now allows you to use them for
   "development purposes". I guess source code counts??
 - I'm fairly sure this is also compatible with the CCPL - there's an
   exception for Minecraft code.

The main motivation for this is to make the Fabric port a little
easier. Hopefully folks (maybe me in the future, we'll see) will no
longer have to deal with mapping hell when merging - only mod loader
hell.

src/main/java/dan200/computercraft/shared/command/UserLevel.java

@@ -61,12 +61,12 @@ public enum UserLevel implements Predicate<CommandSource>
         MinecraftServer server = source.getServer();
         Entity sender = source.getEntity();
 
-        if( server.isSinglePlayer() && sender instanceof PlayerEntity &&
+        if( server.isSingleplayer() && sender instanceof PlayerEntity &&
             ((PlayerEntity) sender).getGameProfile().getName().equalsIgnoreCase( server.getServerModName() ) )
         {
             if( this == OWNER || this == OWNER_OP ) return true;
         }
 
-        return source.hasPermissionLevel( toLevel() );
+        return source.hasPermission( toLevel() );
     }
 }