DiffEngine compatibility issues

[gtbuchanan]

DiffEngine 14 dropped support for Shouldly's current target frameworks netstandard2.0 and net5.0 and added a new optional Encoding argument for DiffRunner.Launch. The latest version of Verify uses DiffEngine 15. This causes the following runtime error when referencing both Shouldly and Verify and calling ShouldMatchApproved:

System.MissingMethodException : Method not found: 'DiffEngine.LaunchResult DiffEngine.DiffRunner.Launch(System.String, System.String)'.
ShouldMatchApprovedTestExtensions.ShouldMatchApproved(String actual, Action`1 configureOptions, String customMessage)

I was initially going to submit a PR to bump DiffEngine but realized this resulted in more drastic changes. In order to support the new DiffEngine and resolve this issue, Shouldly would have to target net472 and net6.0 at a minimum (dropping netstandard altogether). Is this something you're willing to do?