[video_player] Remove default method channel implementation (#6341)

Eliminates `MethodChannelVideoPlayer`, and instead makes the default a
no-op subclass of the interface (i.e., one that throws
`UnimplementedError` for everything). This moves the plugin from option
3 to option 2 in
https://docs.flutter.dev/go/platform-channels-in-federated-plugins.

This implementation is being removed because its use of Pigeon made
ongoing maintenance impractical. Since default implementations are
intended for backward compatibility with third-party native-only
implementations, the platform channel internals of default
implementations are considered part of the API surface. Pigeon considers
those details implementation details, which means it was impossible to
ever update the version of Pigeon used here without breaking changes.
This means that fundamentally, Pigeon is not a viable choice for default
implementations.

The alternative to removing it would be replacing it with a new
non-Pigeon method channel implementation, but that would also be a
breaking change, defeating the purpose of backward compatibility.
This would only benefit new third-party implementations, but new
implementations should implement the platform interface rather than
using the legacy default implementation.

Fixes flutter/flutter#110017

Author: stuartmorgan-g

packages/video_player/video_player_platform_interface/CHANGELOG.md

@@ -1,5 +1,9 @@
-## NEXT
+## 6.0.0
 
+* **BREAKING CHANGE**: Removes `MethodChannelVideoPlayer`. The default
+  implementation is now only a placeholder with no functionality;
+  implementations of `video_player` must include their own `VideoPlayerPlatform`
+  Dart implementation.
 * Updates minimum Flutter version to 2.10.
 * Fixes violations of new analysis option use_named_constants.