Replace Color.withOpacity with Color.withValues (Resolves #2437)

[angelosilvestre]

Replace Color.withOpacity with Color.withValues. Resolves #2437

The CI is failing on the super_text_layout analysis due to Color.withOpacity being deprecated:

Analyzing super_text_layout...                                  

   info • 'withOpacity' is deprecated and shouldn't be used. Use .withValues() to avoid precision loss • example/lib/main.dart:254:71 • deprecated_member_use
   info • 'withOpacity' is deprecated and shouldn't be used. Use .withValues() to avoid precision loss • example/lib/main.dart:4[8](https://github.com/superlistapp/super_editor/actions/runs/12129129760/job/33816940663?pr=2436#step:5:9)4:21 • deprecated_member_use
   info • 'withOpacity' is deprecated and shouldn't be used. Use .withValues() to avoid precision loss • example/lib/main.dart:502:30 • deprecated_member_use
   info • 'withOpacity' is deprecated and shouldn't be used. Use .withValues() to avoid precision loss • lib/src/caret_layer.dart:182:42 • deprecated_member_use

To migrate, all we need is to replace calls to withOpacity to withValues passing the opacity as the alpha argument.

Source: https://docs.flutter.dev/release/breaking-changes/wide-gamut-framework#opacity

[matthew-carroll]

LGTM