chore: Bump xml version (#57)

* bump xml version

* Fix duplicate xml declaration at the test tmx file

Author: Hwan-seok

packages/tiled/pubspec.yaml

@@ -11,7 +11,7 @@ dependencies:
   flutter:
     sdk: flutter
   meta: ^1.7.0
-  xml: ^5.4.1
+  xml: ^6.1.0
 
 dev_dependencies:
   dartdoc: ^4.1.0

packages/tiled/test/fixtures/map_with_multiple_tilesets.tmx

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <map version="1.4" tiledversion="1.4.0" orientation="orthogonal" renderorder="right-down" width="32" height="128" tilewidth="16" tileheight="16" infinite="0" nextlayerid="4" nextobjectid="8">
 <tileset firstgid="1" source="external_tileset_1.tsx"/>
-<?xml version="1.0" encoding="UTF-8"?>
 <tileset version="1.2" tiledversion="2018.11.29" name="level_embed" tilewidth="16" tileheight="16" tilecount="136" columns="17">
     <image source="../images/map-level1.png" width="272" height="128"/>
 </tileset>