flutter: Update copyright year to 2021

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

@@ -47,7 +47,7 @@
   "@daSanQi": {
     "description": "Da San Qi"
   },
-  "copyright": "Copyright © 2019-2020 The Sanmill Authors",
+  "copyright": "Copyright © 2019-2021 The Sanmill Authors",
   "@copyright": {
     "description": "Copyright"
   },

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

@@ -11,7 +11,7 @@
   "ok": "好的",
   "cancel": "取消",
   "daSanQi": "打三棋",
-  "copyright": "版权所有 © 2019-2020 The Sanmill Authors",
+  "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。",
   "tipSelectWrong": "选择的子不对",
   "tipPlace": "请落子",

src/ui/flutter/pubspec.yaml

@@ -1,7 +1,7 @@
 name: sanmill
 description: A new Flutter project.
 
-version: 0.15.0+1318
+version: 0.16.0+1392
 
 environment:
   sdk: ">=2.1.0 <3.0.0"