style(ui): shorten update button label to Update

The shared newVersionUpdate label now reads Update on the app upgrade
dialog, the tracker update button and the extension update dialog, so
the extension flow matches the host dialog wording.

ui/flutter/lib/l10n/app_en.arb

@@ -106,7 +106,7 @@
   "add": "Add",
   "edit": "Edit",
   "newVersionTitle": "Discover new version @version",
-  "newVersionUpdate": "Update Now",
+  "newVersionUpdate": "Update",
   "newVersionLater": "Later",
   "extensions": "Extensions",
   "extensionInstallUrl": "Install URL",

ui/flutter/lib/l10n/app_zh.arb

@@ -99,7 +99,7 @@
   "add": "添加",
   "edit": "编辑",
   "newVersionTitle": "发现新版本 @version",
-  "newVersionUpdate": "立即更新",
+  "newVersionUpdate": "更新",
   "newVersionLater": "稍后再说",
   "extensions": "扩展",
   "extensionInstallUrl": "安装链接",

ui/flutter/test/widget_test.dart

@@ -2329,7 +2329,7 @@ void main() {
     final progress = find.descendant(of: updateButton, matching: find.byType(shad.CircularProgressIndicator));
     expect(progress, findsOneWidget);
     expect(tester.getSize(progress), const Size.square(14));
-    expect(find.descendant(of: updateButton, matching: find.text('Update Now')), findsOneWidget);
+    expect(find.descendant(of: updateButton, matching: find.text('Update')), findsOneWidget);
 
     runtimeController.completeTrackerUpdate();
     await tester.pumpAndSettle();