Add target framework moniker to releases.json file

# Issue Title

Add target framework moniker to releases.json file.

# General

We have a provisioning process that creates new .NET Core apps for our developers. By default, we target the latest .NET Core version at the LTS support level. As part of this process, we create a .csproj file and need to set its `TargetFramework` property. It would be nice if the releases.json file had a property for the target framework moniker for each version.

Currently, we convert the `version-runtime` property to a `Version` object, and use its `Major` and `Minor` properties to create the target framework, e.g. (sample PowerShell)

[version]$runtimeVersion = $releasesJsonVersion.'version-runtime'
$targetFramework = 'netcoreapp{0}.{1}' -f $runtimeVersion.Major,$runtimeVersion.Minor

But I'd prefer to just

$releasesJsonVersion.'target-framework-moniker'