Update rxdart 0.22.6 -> 0.23.1

[BeMacized]

Allows just_audio 0.0.2 to be used with audio_service 0.5.5.

[yringler]

I think you want to run flutter pub get to update pubspec.lock
just_audio already uses 0.23.1, and has that in the lock! Yeah, this is a good idea.

[BeMacized]

Looks like this is now in the current master.