Incorrect definition of GKLocalPlayer.AuthenticateHandler: missing nullables

### Apple platform

iOS

### Framework version

net10.0-*

### Affected platform version

ios SDK 26.2.10191/10.0.100

### Description

The definition of `AuthenticateHandler` in GKLocalPlayer.cs is incorrect:

```c#
public virtual Action? AuthenticateHandler
```

Given the [doc](https://developer.apple.com/documentation/gamekit/gklocalplayer/authenticatehandler?language=objc) both UIViewController and NSError are nullable.

So it should be:
```C#
public virtual Action? AuthenticateHandler
```

### Steps to Reproduce

N/A

### Did you find any workaround?

Ignore the warning.

### Build logs

_No response_