Convert RCTTiming to TurboModule (#57037)

Summary:

Wires `RCTTiming` (iOS CoreModule for `setTimeout`/`setInterval`/idle callbacks) to the existing TurboModule spec at `packages/react-native/src/private/specs_DEPRECATED/modules/NativeTiming.js`. `RCTTiming` was the only remaining `legacy = True` entry in `React/CoreModules/BUCK`. The existing `RCT_EXPORT_METHOD` signatures already match the spec exactly, so this is a registration/wiring change only.

Changes:
- `RCTTiming.mm`: declare `<NativeTimingSpec>` conformance in a private interface and add `getTurboModule:` returning `NativeTimingSpecJSI`, mirroring the pattern used by `RCTAppState`.
- `React/CoreModules/BUCK`: drop `legacy = True` from the `Timing` entry of `react_module_plugin_providers`.

No public header changes — the bridgeless path (`ObjCTimerRegistry`) continues to instantiate `RCTTiming` via `initWithDelegate:` and the `RCTTimingDelegate` protocol. No CocoaPods / Swift Package changes are needed: `React-CoreModules.podspec` globs all `CoreModules/*.mm` and `FBReactNativeSpec` codegens `NativeTimingSpecJSI` from the existing JS spec.

## Changelog: 
[IOS][Fixed]Convert RCTTiming to TurboModule

Differential Revision: D107201906

Author: christophpurrer

packages/react-native/React/CoreModules/RCTTiming.mm

@@ -95,6 +95,9 @@ - (void)timerDidFire
 
 @end
 
+@interface RCTTiming () <NativeTimingSpec>
+@end
+
 @implementation RCTTiming {
   NSMutableDictionary<NSNumber *, _RCTTimer *> *_timers;
   NSTimer *_sleepTimer;
@@ -427,6 +430,12 @@ - (void)createTimerForNextFrame:(nonnull NSNumber *)callbackID
   }
 }
 
+- (std::shared_ptr<facebook::react::TurboModule>)getTurboModule:
+    (const facebook::react::ObjCTurboModule::InitParams &)params
+{
+  return std::make_shared<facebook::react::NativeTimingSpecJSI>(params);
+}
+
 @end
 
 Class RCTTimingCls(void)