Removed different method signatures for NETFRAMEWORK and NETSTANDARD2_0

[Ergamon]

The CallerArgumentExpressionAttribute also works for older versions than NETSTANDARD2_1.

So I changed the compiler directives accordingly and removed all differing signatures.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Ardalis.GuardClauses	47%	92%	287	❌
Summary	47% (270 / 571)	92% (120 / 130)	287	❌

[ardalis]

Thanks! I'll try and get this published this week.