Resolve beta channel deprecation warnings

[cpswan]

Which at_widget package you find issue with?

at_common_flutter, at_contacts_flutter, at_contacts_group_flutter, at_follows_flutter, at_location_flutter, at_login_flutter, at_onboarding_flutter, at_sync_ui_flutter, at_theme_flutter

On what platforms did you experience the issue?

Linux

What version are you using?

Trunk

What happened?

See various failed beta static analysis e.g. https://github.com/atsign-foundation/at_widgets/actions/runs/10882217684/job/30192778788?pr=904

Steps to reproduce

1. Any PR that runs the matrix of static analysis tests

Additional info

No response

Relevant log output

Analyzing at_common_flutter...                                  

   info • 'withOpacity' is deprecated and shouldn't be used. Use .withValues() to avoid precision loss • example/lib/main.dart:139:34 • deprecated_member_use
   info • 'withOpacity' is deprecated and shouldn't be used. Use .withValues() to avoid precision loss • example/lib/main.dart:140:34 • deprecated_member_use

Flutter analyze output

No response

Flutter doctor output

No response