Retire VS 2019 and Windows 8.1 support

Visual Studio 2019 reached end of main-stream support in April 2024.

Visual Studio 2026 support has been added, and the VS 2026 compiler no longer supports Windows 8.1 or earlier. Windows 8.1 reached end of extended support in January 2023.

This also means deprecating the *directxmesh_desktop_2019* NuGet package. The *directxmesh_desktop_win10* package is built with VS 2022.