fix: Deprecated warnings fixed

[CurtlyCritchlow]

Fixes #905

- What I did

• Replace depreciated withOpacity method with withValues in at_common_flutter, at_onboarding_flutter.
  These fixes on the beta channel results in errors on the stable channel.

- How I did it

- How to verify it

- Description for the changelog

[cpswan]

Thanks @CurtlyCritchlow

My inclination here is to hold off on making these changes until Flutter 3.25 is stable. We have --no-fatal-infos for the tests that run against stable. So the deprecation warnings won't cause stable channel static analysis to fail. We can then make changes that will work for both 3.25 and 3.26 as they'll both support withvalues.

👀 @gkc @XavierChanth