Upgrade NuGet.Frameworks to fix loading projects with the 7.0.4xx SDK

The 7.0.4xx SDKs ship with a newer version of NuGet.Frameworks than
we are shipping. Due to a complicated set of interactions of how
the various AssemblyLoadingContexts work (or don't work as expected!),
this means that when our LSP server uses MSBuildWorkspace to load
projects, it'll end up using the NuGet.Frameworks we use here rather
than the MSBuild one, causing load issues.

The underlying problem is tracked with
dotnet#61454, although I still need
to write up the full situation in that bug since back in June we ended
up running into a similar issue and had lots of deep analysis of it.
Fixing the problem requires changes in several different repositories
(and all involving AssemblyLoadContexts, so the changes are very
technical) so for now we'll just bump the version.

Fixes dotnet/vscode-csharp#5980

eng/Versions.props

@@ -227,7 +227,7 @@
       is expected by the NET SDK used in the Workspace.MSBuild UnitTests. In order to test against the same verion of NuGet
       as our configured SDK, we must set the version to be the same.
      -->
-    <NuGetCommonVersion>6.7.0-preview.2.50</NuGetCommonVersion>
+    <NuGetCommonVersion>6.7.0-rc.127</NuGetCommonVersion>
     <NuGetConfigurationVersion>$(NuGetCommonVersion)</NuGetConfigurationVersion>
     <NuGetFrameworksVersion>$(NuGetCommonVersion)</NuGetFrameworksVersion>
     <NuGetPackagingVersion>$(NuGetCommonVersion)</NuGetPackagingVersion>