New images for .NET Framework 4.8

# New images for .NET Framework 4.8

New images have been released with support for [.NET Framework 4.8 runtime and SDK](https://devblogs.microsoft.com/dotnet/announcing-the-net-framework-4-8/).

## Discussion
Please share your thoughts on this change and see what others are saying at:

* [Microsoft/dotnet-framework-docker#271](https://github.com/Microsoft/dotnet-framework-docker/issues/271)

## Details
* Pull request: #270

You can access these new images with the following tags:
* `mcr.microsoft.com/dotnet/framework/runtime:4.8`
* `mcr.microsoft.com/dotnet/framework/sdk:4.8`

In addition, the `latest` tag has been updated to refer to these new 4.8 images.

Check out the readme files for more information on the latest tags:
* [Runtime](https://github.com/Microsoft/dotnet-framework-docker/blob/master/README.runtime.md)
* [SDK](https://github.com/Microsoft/dotnet-framework-docker/blob/master/README.sdk.md)

The [3.5 SDK](https://github.com/Microsoft/dotnet-framework-docker/tree/master/3.5/sdk) images are intended to always contain the latest .NET Framework SDK and have also been updated to use the 4.8 SDK.