Fix creating worlds with "invalid" names (Fixes #5331)

Spigot-API-Patches/Implement-Keyed-on-World.patch

@@ -98,7 +98,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 -            throw new IllegalArgumentException("World name cannot be null");
 -        }
 +        // Paper start
-+        this(name, NamespacedKey.minecraft(name));
++        this(name, NamespacedKey.minecraft(name.toLowerCase(java.util.Locale.ENGLISH)));
 +    }
  
 -        this.name = name;