4.7.x .NET Framework SDK Docker images no longer published

# 4.7.x .NET Framework SDK Docker images no longer published

The [4.7.x .NET Framework SDK Docker images](https://hub.docker.com/_/microsoft-dotnet-framework-sdk) will no longer be published after Oct. 8, 2019. The 4.8 image tags should be used as a replacement.

## Discussion

You can share your thoughts on this change or see what others are saying at:

* [microsoft/dotnet-framework-docker#428](https://github.com/microsoft/dotnet-framework-docker/issues/428)

## Details

Any references to the following SDK tags should be updated to reference `4.8` instead:
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.1`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.1-windowsservercore-ltsc2016`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.1--windowsservercore-ltsc2016`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.2`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.2-windowsservercore-ltsc2016`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.2--windowsservercore-ltsc2016`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.2-windowsservercore-1803`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.2--windowsservercore-1803`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.2-windowsservercore-ltsc2019`
* `mcr.microsoft.com/dotnet/framework/sdk:4.7.2--windowsservercore-ltsc2019`

If you haven't migrated to using images from [MCR](https://github.com/dotnet/announcements/issues/101) and are still using the .NET Framework repos on Docker Hub, you will need to update your tag references to MCR. This is because the 4.8 images only exist on MCR. These are the following Docker Hub tags that are affected and should be updated to reference the [4.8 tags at the MCR location](https://hub.docker.com/_/microsoft-dotnet-framework-sdk):
* `microsoft/dotnet-framework:4.7.1-sdk`
* `microsoft/dotnet-framework:4.7.1-sdk-windowsservercore-ltsc2016`
* `microsoft/dotnet-framework:4.7.2-sdk`
* `microsoft/dotnet-framework:4.7.2-sdk-windowsservercore-ltsc2016`
* `microsoft/dotnet-framework:4.7.2-sdk-windowsservercore-1803`
* `microsoft/dotnet-framework:4.7.2-sdk-windowsservercore-ltsc2019`