Rebuild downstream .NET QA projects with Visual Studio 2019 (or MSBuild 16)

We need to rebuild our downstream .NET QA test projects with Visual Studio 2019 (or MSBuild 16) to be up to date with Security Code Scan 5.x

The following projects need to be updated, or verified that they're up to date.

• gitlab-org/security-products/tests/csharp-dotnetcore-multiproject (including multiple branches)
• gitlab-org/security-products/tests/dotnet5
• gitlab-org/security-products/tests/webgoat.net