Upgrade from netstandard1.6 to netstandard2.0

[bergmeister]

As discussed in PR 870 here, @JamesWTruher prefers to move to netstandard2.0 to match it closer to the PowerShell Core repo for consistency that uses netcore2.0, which is practically speaking very similar to netstandard2.0
This has the only disadvantage that netstandard2.0 is not implemented by netcore 1.x runtimes and reduces the number of devices on which it can run under due to the vast increase of the APIs. However, since the usage of PowerShell products currently only applies to devices that have the netcore of full .net runtime (but not mobile runtimes such as Mono), this should not be an issue and reduce its practical usage. Therefore for simplicity and the ability of having more APIs, the plan is to perform the upgrade, which should help development due to the API increase and re-evaluate the netstandard question again until there is actually a use case where a lower netstandard is required.

[bergmeister]

From here