Suggestion: Remove netstandard2.1 target, leaving only netstandard2.0

[augustoproiete]

To be aligned with new guidance/best practices on developing Modules for Cake:

§2.6 Avoid targeting additional frameworks such as net461 or net5.0.

Why? Due to current limitations on how modules are loaded, Cake attempts to load all assemblies included in the NuGet package of the module (of all targets) which causes warnings to be displayed and the impression that the module was not loaded and/or is not working.
Once those limitations are fixed, we'll update this recommendation to align with the recommendations for Cake Addins.