feat!: remove onTimingsCallback for Flutter 3.0

[wolfenrain]

Description

This technically speaking is a breaking change, but since we are already planning on deprecating the FPSCounter the team has internally decided to turn the fps() method into a copy of the functionality from #1595 so that we can remove the method onTimingsCallback already so Flutter 3.0 users wont have any warnings. This method is either not used or a really small portion of people are using it, compared to the FPSCounter.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples.

Breaking Change

• Yes, this is a breaking change, but as we are doing this on purpose I have not added the suffix !.
• No, this is not a breaking change.

Related Issues

[st-pasha]

👍 to this change - even with the old FpsCounter it was unclear why the method was exposed in the Game's API, since it was possible to subscribe to the timing events from the mixing itself.

this is a breaking change, but as we are doing this on purpose I have not added the suffix !.

That doesn't sound right though. All breaking changes that we do are deliberate, and are marked with !. Of course, there could be some breaking changes that we introduced without noticing, by accident - those will have no !.