[dotnet, csharp] Run tests for supported platforms .NET 6, .NET Framework 4.6.2 (#12378)

What's changed?
- Update some nuget packages (for test projects)
- Run unit tests for supported platforms: .NET 6 + .NET Framework 4.6.2

Notes:
.NET Framework 4.6.1 is out of support, .NET Framework 4.6.2 is the older version that is still supported
https://devblogs.microsoft.com/dotnet/net-framework-4-5-2-4-6-4-6-1-will-reach-end-of-support-on-april-26-2022/

.NET 5 is out of support
https://devblogs.microsoft.com/dotnet/dotnet-5-end-of-support-update/

.NET Core 3.1 is out of support
https://devblogs.microsoft.com/dotnet/net-core-3-1-will-reach-end-of-support-on-december-13-2022/

Visual studio 2012 is out of support
https://devblogs.microsoft.com/visualstudio/support-ends-for-older-versions-of-visual-studio-feb2022

Closes #12378

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12378 from Romfos:main 227661e02cc47673295a5d0e03763f8e2bf0f124
PiperOrigin-RevId: 523196896