Upcoming end of support for .NET Core 3.1 and .NET Framework 4.6.1

[cobya]

As announced previously in #386, credprovider compatibility with .NET Core 3.1 will be ending as it has been out of support since December 13, 2022. Compatibility with .NET Framework 4.6.1 will also be removed as support ended on April 26, 2022.

For more information on .NET release and support schedules, please see https://github.com/microsoft/artifacts-credprovider?tab=readme-ov-file#release-version-200. To continue using credprovider, please update your .NET versions to .NET Framework 4.8.1 or .NET 8 as mentioned in the README.