Stride samples/4.4.0 Released

Stride samples/4.4.0, released for Stride 4.4.0-beta8, is a broad sample and template refresh with a lot of build-system cleanup behind it. The biggest practical changes are the new 2D Platformer template, expanded dotnet new asset packs, and a migration to .slnx plus Stride.Build.Sdk. Developers also get numerous sample fixes, test harness upgrades, and a few crash/serialization corrections.