Upgrade to medea-flutter-webrtc 0.12.0

[evdokimovs]

Related to #194

Synopsis

New version of medea-flutter-webrtc was released, so we need to update it here.

Also along side with medea-flutter-webrtc , the new version flutter-rust-bridge was released, so we need to update it too.

Checklist

• Created PR:
  • In draft mode
  • Name contains issue reference
  • Has type and k:: labels applied
  • Has assignee
• Before review:
  • Documentation is updated (if required)
  • Tests are updated (if required)
  • Changes conform code style
  • CHANGELOG entry is added (if required)
  • FCM (final commit message) is posted or updated
  • Draft mode is removed
• Review is completed and changes are approved
  • FCM (final commit message) is approved
• Before merge:
  • Milestone is set
  • PR's name and description are correct and up-to-date
  • All temporary labels are removed

[evdokimovs]

FCM

Upgrade `medea_flutter_webrtc` package to 0.12.0 version (#195)

- upgrade `flutter-rust-bridge` crate to 2.7.0 version
- switch back to upstream `dart-sys` crate