Update Prerequisites topics for .NET Core 2.2

[mairaw]

• Windows: https://docs.microsoft.com/en-us/dotnet/core/windows-prerequisites
• macOS: https://docs.microsoft.com/en-us/dotnet/core/macos-prerequisites
• Linux: https://docs.microsoft.com/en-us/dotnet/core/linux-prerequisites

[Thraka]

Status update
Following up with author of https://github.com/dotnet/core/blob/master/release-notes/2.2/2.2-supported-os.md to confirm there are no changes to prereqs for 2.2

[mairaw]

I just checked the Windows version and is not fully up-to-date. I'll create a PR shortly with some more changes to address this. I'll check the others later.

[mairaw]

@Andrew-MSFT @KathleenDollard @richlander I don't seem to be able to select the Target Framework version from the Build menu on VS 15.9.3 anymore. By design?

I'm updating the screenshots we have at https://docs.microsoft.com/en-us/dotnet/core/windows-prerequisites#prerequisites-with-visual-studio-2017.