VMR Stage 2 Builds Failing in Nuget Client with Obsolete Reference

[ellahathaway]

See #4494. Similar error now appearing in nuget-client.

While testing the changes in dotnet/arcade#14919, a new error appeared in the stage 2 build:

/vmr/src/nuget-client/src/NuGet.Core/NuGet.Configuration/Settings/Items/FileClientCertItem.cs(170,66): error SYSLIB0057: 'X509Certificate2.X509Certificate2(string)' is obsolete: 'Loading certificate data through the constructor or Import is obsolete. Use X509CertificateLoader instead to load certificates.' (https://aka.ms/dotnet-warnings/SYSLIB0057) [/vmr/src/nuget-client/src/NuGet.Core/NuGet.Configuration/NuGet.Configuration.csproj]

This error is caused by dotnet/runtime#104165.