Build from source instructions need to be updated for VS 2026

Instructions here: https://github.com/dotnet/aspnetcore/blob/main/docs/BuildFromSource.md

Specifically, the [InstallVisualStudio.ps1](https://github.com/dotnet/aspnetcore/blob/main/eng/scripts/InstallVisualStudio.ps1) script needs to be updated to support 2026. It only allows VS 2022.