Fix TargetFrameworks

[DorianGreen]

This PR normalizes the TargetFrameworks in all the projects. Addresses #148

• Projects and tests that do not require AspNetCore should target netstandard2.0. since there are no advanced language features there is no need to multi-target with net6+
• Projects and tests that that require AspNetCore multi-target net6;net7. this is to support users who cannot upgrade easily. once net8 is released, we should add it to the list and potentially remove net7 at some future date.
• Test projects multi-target net4.7.1;net6;net7. this is to ensure all cases pass on all supported frameworks.

notes: some of the referenced packages require per target version overrides in order to compile. caution should be exercised when updating dependencies. it is best to manually update the proj/props files.

This PR also adds some GlobalPackageReferences. PolySharp can be used to access modern language polyfills, and is a dev only reference SourceLink will help clients with debugging.

[DorianGreen]

net4.7.1 tests are failing. for details on the issue see: https://stackoverflow.com/questions/47155559/could-not-load-file-or-assembly-microsoft-visualstudio-testplatform-objectmodel

At this point we can decide to try and fix the build or exclude net4.7.1 as a Test target. note that all the tests pass locally.

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Ardalis.Result.Sample.Core	18%	18%	63	❌
Ardalis.Result.FluentValidation	86%	50%	6	✔
Ardalis.Result	20%	0%	82	❌
Summary	23% (94 / 416)	17% (9 / 54)	151	❌

[DorianGreen]

Tests pass by explicitly adding the failing package, and adding all missing binding redirects into app.config of unit tests.

[ardalis]

Looks good, thanks!