Swap #pragma with NoWarn in suppression sections

[gewarren]

          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 <NoWarn> list is safe and most desirable.

Originally posted by @terrajobst in #43119 (comment)

[terrajobst]

To clear, that guidance is specific to Obsolete, Preview, and Experimental -- I wouldn't usually recommend that for compiler warnings.