ci: Bump Flutter to 3.16.0

[witwash]

Summary

• bumped GH action Flutter version
• fixed migration issues

Testing steps

• WillPopScope is deprecated and replaced with the PopScope. There is an issue raised asking to preserve WillPopScope, but the outcome is not clear. Need to test if the back button press is acting as intended.

Follow-up issues

• Material 3 is now enabled by default and we need to test if it is not bleeding through our component and everything is looking like it should. There is a possibility to disable Material 3 for the time, but Material 2 is going to be deprecated so it would just postpone possible issues.
• feat!: Add new rules from Dart 3.2 #474

Check during review

• Verify against Jira issue.
• Is the PR over 300 additions? Consider rejecting it with advice to split it. Is it over 500 additions? It should definitely be rejected.
• Unused code removed.
• Build passing.
• Is it a bug fix? Check that it is covered by a proper test (unit or integration).

[ookami-kb]

There are also some lint rules updated in Dart 3.2, let's include them as well (or maybe in a separate PR?).

[witwash]

@ookami-kb rules are in separate PR: #474

[witwash]

@ookami-kb, ping

[ookami-kb]

@witwash please update PR title to include the type. Also, would it be a breaking change to optimus?

[witwash]

@ookami-kb, didn't notice, thanks for noting. Done 👍