warning "retain (or strong)' attribute on property 'window' does not match the property inherited from 'UIApplicationDelegate' [-Wproperty-attribute-mismatch]" after building ios xamarin app

### Steps to reproduce:

1. Create iOS Single view App
2. Build for device

### Expected:

There are no warnings

### Actual:

1. After building project, there are two warnings

```
In file included from /Users/rolf/Projects/devdiv1283065/obj/iPhone/Debug/mtouch-cache/registrar.m:3:
/Users/rolf/Projects/devdiv1283065/obj/iPhone/Debug/mtouch-cache/registrar.h(20,43): warning GBD8E3C1A: 'retain (or strong)' attribute on property 'window' does not match the property inherited from 'UIWindowSceneDelegate' [-Wproperty-attribute-mismatch] [/Users/rolf/Projects/devdiv1283065/devdiv1283065.csproj]
@property (nonatomic, assign) UIWindow * window;
^
/Users/rolf/Projects/devdiv1283065/obj/iPhone/Debug/mtouch-cache/registrar.h(41,43): warning GBD8E3C1A: 'retain (or strong)' attribute on property 'window' does not match the property inherited from 'UIApplicationDelegate' [-Wproperty-attribute-mismatch] [/Users/rolf/Projects/devdiv1283065/devdiv1283065.csproj]
@property (nonatomic, assign) UIWindow * window;
```

This happens because the static registrar doesn't take into account inherited properties when generating the Objective-C property declaration.

Full build log:
[build.txt](https://github.com/xamarin/xamarin-macios/files/6041904/build.txt)

Project:
[devdiv1283065-d8864fc.zip](https://github.com/xamarin/xamarin-macios/files/6041912/devdiv1283065-d8864fc.zip)

Source: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1283065