[iOS] Fix FlutterViewController retain cycle (flutter/engine#43379)

Fixes [flutter/flutter#126560](https://github.com/flutter/flutter/issues/126560)

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -2190,7 +2190,7 @@ static BOOL IsSelectionRectBoundaryCloserToPoint(CGPoint point,
     NSMutableDictionary<NSString*, FlutterTextInputView*>* autofillContext;
 @property(nonatomic, retain) FlutterTextInputView* activeView;
 @property(nonatomic, retain) FlutterTextInputViewAccessibilityHider* inputHider;
-@property(nonatomic, readonly) id<FlutterViewResponder> viewResponder;
+@property(nonatomic, readonly, weak) id<FlutterViewResponder> viewResponder;
 @end
 
 @implementation FlutterTextInputPlugin {

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

@@ -1055,6 +1055,16 @@ extern NSNotificationName const FlutterViewControllerWillDealloc;
   OCMVerify([self.mockEngine binaryMessenger]);
 }
 
+- (void)testViewControllerIsReleased {
+  __weak FlutterViewController* weakViewController;
+  @autoreleasepool {
+    FlutterViewController* viewController = [[FlutterViewController alloc] init];
+    weakViewController = viewController;
+    [viewController viewDidLoad];
+  }
+  XCTAssertNil(weakViewController);
+}
+
 #pragma mark - Platform Brightness
 
 - (void)testItReportsLightPlatformBrightnessByDefault {