Add obsolete warnings

[scottdorman]

We should [Obsolete] the ValidState API with a message telling folks to use Verify.Operation instead.

That is similarly redundant with Requires.Argument and should be deprecated. Having redundant methods for the sake of sounding "fluent" makes an API more confusing, IMO.

Originally posted by @AArnott in #49 (comment)

[scottdorman]

Should the unit tests be removed for ValidState and That or should they be wrapped in a pragma to ignore the CS0618 warning?