Update experimental/web_dashboard

[domesticmouse]

From failing CI run:

== Testing 'experimental/web_dashboard' on Flutter's beta channel ==
~/work/samples/samples/experimental/web_dashboard ~/work/samples/samples
Running "flutter pub get" in web_dashboard...                       3.7s
Analyzing web_dashboard...                                      

   info • 'showSnackBar' is deprecated and shouldn't be used. Use ScaffoldMessenger.showSnackBar. This feature was deprecated after v1.23.0-14.0.pre. • lib/src/pages/entries.dart:149:38 • deprecated_member_use
   info • 'showSnackBar' is deprecated and shouldn't be used. Use ScaffoldMessenger.showSnackBar. This feature was deprecated after v1.23.0-14.0.pre. • lib/src/pages/sign_in.dart:98:26 • deprecated_member_use
   info • 'title' is deprecated and shouldn't be used. Use "label" instead, as it allows for an improved text-scaling experience. This feature was deprecated after v1.19.0. • lib/src/widgets/third_party/adaptive_scaffold.dart:143:15 • deprecated_member_use