Deprecate RawKeyEvent, RawKeyboard, et al. (#136677)

## Description

This starts the deprecation of the `RawKeyEvent`/`RawKeyboard` event system that has been replaced by the `KeyEvent`/`HardwareKeyboard` event system.

Migration guide is available here: https://docs.flutter.dev/release/breaking-changes/key-event-migration

## Related Issues
 - flutter/flutter#136419

## Related PRs
 - flutter/website#9889

Author: gspencergoog

examples/api/lib/widgets/hardware_keyboard/key_event_manager.0.dart

@@ -5,6 +5,10 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
+// TODO(gspencergoog): Delete this example when deprecated RawKeyEvent API is
+// removed.
+// ignore_for_file: deprecated_member_use
+
 /// Flutter code sample for [KeyEventManager.keyMessageHandler].
 
 void main() {

packages/flutter/lib/src/services/binding.dart

@@ -16,6 +16,7 @@ import 'debug.dart';
 import 'hardware_keyboard.dart';
 import 'message_codec.dart';
 import 'platform_channel.dart';
+import 'raw_keyboard.dart' show RawKeyboard;
 import 'restoration.dart';
 import 'service_extensions.dart';
 import 'system_channels.dart';
@@ -66,6 +67,13 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
 
   /// The global singleton instance of [KeyEventManager], which is used
   /// internally to dispatch key messages.
+  ///
+  /// This property is deprecated, and will be removed. See
+  /// [HardwareKeyboard.addHandler] instead.
+  @Deprecated(
+    'No longer supported. Add a handler to HardwareKeyboard instead. '
+    'This feature was deprecated after v3.18.0-2.0.pre.',
+  )
   KeyEventManager get keyEventManager => _keyEventManager;
   late final KeyEventManager _keyEventManager;
 
@@ -89,8 +97,8 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
   BinaryMessenger get defaultBinaryMessenger => _defaultBinaryMessenger;
   late final BinaryMessenger _defaultBinaryMessenger;
 
-  /// A token that represents the root isolate, used for coordinating with background
-  /// isolates.
+  /// A token that represents the root isolate, used for coordinating with
+  /// background isolates.
   ///
   /// This property is primarily intended for use with
   /// [BackgroundIsolateBinaryMessenger.ensureInitialized], which takes a

packages/flutter/lib/src/services/debug.dart

@@ -13,6 +13,12 @@ export 'hardware_keyboard.dart' show KeyDataTransitMode;
 /// Setting [debugKeyEventSimulatorTransitModeOverride] is a good way to make
 /// certain tests simulate the behavior of different type of platforms in terms
 /// of their extent of support for keyboard API.
+///
+/// This value is deprecated and will be removed.
+@Deprecated(
+  'No longer supported. Transit mode is always key data only. '
+  'This feature was deprecated after v3.18.0-2.0.pre.',
+)
 KeyDataTransitMode? debugKeyEventSimulatorTransitModeOverride;
 
 /// Setting to true will cause extensive logging to occur when key events are