Current versions of the Windows desktop build files don't require a specific Windows 10 SDK version, but doctor still checks for one since vswhere doesn't allow for flexible queries. This has been a common source of issues for people setting up on Windows for the first time, because the current VS installer by default only includes a newer version of the SDK than what doctor is looking for.

This removes the vswhere SDK check, and instead uses a manual check for SDKs. Since this uses undocumented (although fairly widely used, so relatively unlikely to change) registry information, the check is non-fatal, so that builds can progress even if the SDK isn't found by doctor; in practice, it's very unlikely that someone would install the C++ Windows development workload but remove the selected-by-default SDK from the install.

Now that all requirements are default, the instructions when missing VS have been simplified so that they no longer list individual components, and instead just say to include default items.

Fixes #50487