feat!: Kill MultiTouchTapDetector and MultiTouchDragDetector

[luanpotter]

Description

Kill MultiTouchTapDetector and MultiTouchDragDetector in favour of the existing TapCallbacks and DragCallbacks (already supported on both component and flame game level). The pointerId that used to be a callback parameter is now carried on the event (event.pointerId), so multi-touch tracking is unchanged.

The MultiTouchDragDetector/PanDetector conflict assert in GameWidget can die too, along with the note in gesture_input.md explaining that advanced detectors always win the gesture arena (MultiDragScaleDispatcher is our new shape and natively handles this under the hood).

Note that while TapCallbacks has no MultiTouchTapDetector.onTap, Flutter's "tap completed" callback, you can use onTapUp instead.

Also cleans up tangent things, such as the game's MultiTapListener branch in initializeGestures and the matching clause in containsEventHandlerAt, which are now unreachable; the docs for legacy are updated, with generic sessions (which were already referencing other dead code) just shifting to other (still existent) legacy examples (we could just delete it, but I'd rather keep deleting piece-meal with each PR for absolute clarity), and finally general reframing tweaks on that doc around the legacy vs new system.

Finally this is adding the migration entry for the prior removal that I forgot (:facepalm:), alongside this one's.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• 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 or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.