NuGet dependency version issue for Microsoft.Extensions.Logging.Abstractions
## 🐛 Bug Report

Looking at this
https://github.com/MvvmCross/MvvmCross/blob/92621bdb48dc3f3d0c0e1a0630568d662d678e63/Directory.Packages.props#L42-L43

In MvvmCross `Microsoft.Extensions.Logging.Abstractions` is set to 5.0.0 and `Serilog.Extensions.Logging` is set to 3.0.1

However the dependency graph of `Serilog.Extensions.Logging` is
[Serilog.Extensions.Logging](https://www.nuget.org/packages/Serilog.Extensions.Logging/3.0.1) ->
[Microsoft.Extensions.Logging](https://www.nuget.org/packages/Microsoft.Extensions.Logging/2.0.0) ->
Microsoft.Extensions.Logging.Abstractions (>= 2.0.0)

This seems to be causing issues

Get the following error when trying to build
```
Can't resolve the reference 'Microsoft.Extensions.Logging.Abstractions.Internal.NullScope Microsoft.Extensions.Logging.Abstractions.Internal.NullScope::get_Instance()', referenced from the method 'System.IDisposable Microsoft.Extensions.Logging.Logger::BeginScope(TState)' in 'Microsoft.Extensions.Logging.Abstractions, Version=2.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'.
```

Locally (Windows) it works for iPhoneSimulator and Android but not on iPhone
On build server (Mac) it fails for both iPhone and Android

I've managed to work around by adding specifically versioned references to the intermediate dependencies but I get build warnings and have concerns that I might get runtime errors.

Could the `Microsoft.Extensions.Logging.Abstractions` dependency be set to `>=2.0.0` like `Serilog.Extensions.Logging` or are later features required?

### Expected behavior
Project builds

### Reproduction steps
1. Open [TipCalc](https://github.com/MvvmCross/MvvmCross-Samples/tree/master/TipCalc)
1. Select `TipCalc.iOS` project
1. Set platform to `iPhone`
1. Build

### Configuration

**Version:** 8.x

**Platform:** 
- [ X] :iphone: iOS
- [ ] :robot: Android
