Swap #pragma with NoWarn in suppression sections

I suggest to swap this with the next section:

* `#pragma` is fairly ugly and requires touching multiple places in code.
* Diagnostic IDs are meant to be scoped. Putting them in the project-global `` list is safe and most desirable.

_Originally posted by @terrajobst in https://github.com/dotnet/docs/pull/43119#discussion_r1811567034_