flutter: Change game warning text font size to 11 and align center

src/ui/flutter/lib/l10n/intl_en.arb

@@ -51,7 +51,7 @@
   "@copyright": {
     "description": "Copyright"
   },
-  "gameWarning": "              Healthy Gaming Advisory\nBoycott harmful games; reject game piracy.\nExercise self-protection; avoid deception.\nModeration promotes brain health; excess play is harmful to the body.\nWell-planned use of your time will lead to a healthy lifestyle.",
+  "gameWarning": "Healthy Gaming Advisory\nBoycott harmful games; reject game piracy.\nExercise self-protection; avoid deception.\nModeration promotes brain health; excess play is harmful to the body.\nWell-planned use of your time will lead to a healthy lifestyle.",
   "@gameWarning": {
     "description": "Game Warning"
   },

src/ui/flutter/lib/l10n/intl_zh.arb

@@ -12,7 +12,7 @@
   "cancel": "取消",
   "daSanQi": "打三棋",
   "copyright": "版权所有 © 2019-2021 The Sanmill Authors",
-  "gameWarning": "              健康游戏忠告\n抵制不良游戏，拒绝盗版游戏。\n注意自我保护，谨防受骗上当。\n适度游戏益脑，沉迷游戏伤身。\n合理安排时间，享受健康生活。",
+  "gameWarning": "健康游戏忠告\n抵制不良游戏，拒绝盗版游戏。\n注意自我保护，谨防受骗上当。\n适度游戏益脑，沉迷游戏伤身。\n合理安排时间，享受健康生活。",
   "tipSelectWrong": "选择的子不对",
   "tipPlace": "请落子",
   "tipBanPlace": "不能落在此处",

src/ui/flutter/lib/widgets/main_menu.dart

@@ -141,7 +141,8 @@ class _MainMenuState extends State<MainMenu> with TickerProviderStateMixin {
           ),
           Expanded(child: SizedBox()),
           Text(S.of(context).gameWarning,
-              style: TextStyle(color: Colors.black54, fontSize: 16)),
+              style: TextStyle(color: Colors.black54, fontSize: 11),
+              textAlign: TextAlign.center),
           Expanded(child: SizedBox()),
           Text(S.of(context).copyright,
               style: TextStyle(color: Colors.black54, fontSize: 16)),