Prepare Internal Release Branch

When preparing a public branch for internal release, apply the following changes:

1. Directory.Build.props

Add NU1507 warning suppression after the TestNetCoreTargetFrameworks PropertyGroup. Internal branches don't use package source mapping due to internal feeds:

<!-- Internal branches don't use package source mapping feature due to internal feeds, so disable NU1507 warning saying it should be used. -->
<PropertyGroup>
  <NoWarn>$(NoWarn);NU1507</NoWarn>
</PropertyGroup>

Insert this new PropertyGroup right after the closing </PropertyGroup> that contains TestNetCoreTargetFrameworks.

2. NuGet.config