Remove class constraint from Guard.Is[Not]Null APIs

[Sergio0694]

Closes #43

This PR removes the where T : class constraint from the Guard.Is[Not]Null APIs.

API diff

 namespace CommunityToolkit.Diagnostics;

 public static class Guard
 {
     public static void IsNull<T>(T? value, [CallerArgumentExpression("value")] string name = "")
-        where T : class

     public static void IsNotNull<T>([NotNull] T? value, [CallerArgumentExpression("value")] string name = "")
-        where T : class
 }