Update platform_channels for upcoming release

[domesticmouse]

From failing CI run:

== Testing 'platform_channels' on Flutter's beta channel ==
~/work/samples/samples/platform_channels ~/work/samples/samples
Running "flutter pub get" in platform_channels...                  888ms
Analyzing platform_channels...                                  

   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/method_channel_demo.dart:87:26 • 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/pet_list_screen.dart:28:34 • 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/pet_list_screen.dart:96:26 • deprecated_member_use

[AyushBherwani1998]

Happy to help and update sample @domesticmouse :)

[domesticmouse]

Thank you @AyushBherwani1998, PRs always gratefully accepted =)

[redbrogdon]

Relevant breaking change doc: https://flutter.dev/docs/release/breaking-changes/scaffold-messenger

Good to see you again, @AyushBherwani1998! I hope you're doing well.

[AyushBherwani1998]

Hey @redbrogdon. Thanks, I'm doing good. I hope you are doing well too.

[redbrogdon]

I've brought the beta branch up to date so this can be landed there.

[adarsh-technocrat]

Hello @domesticmouse as related PR has been to merge to the beta branch we can close this issue :)

[domesticmouse]

Thank you again 😎