Change target frameworks to net461 and netstandard2.0

[gitfool]

Given a netstandard2.0 TFM we should be able to run scripts using dotnet cake.

I'll have a crack and submit a PR... this is trickier than I thought due to its dependencies.

[AdaskoTheBeAsT]

Cannot be done as netstandard2.0 - needed to be targeted to concrete versions o framework - done in #83

[A9G-Data-Droid]

The last a version that targets netstandard2 is Powershell 6.1.2 but that version doesn't come with the OS, like framework does.

If the draw to framework is that it comes with a base OS install then you would want a netstandard version that targets 'PSVersion 5.1.19041.4170' as that is the current version shipped with Win10.

I would recommend having separate versions for PowerShell5\netstandard and latest\latest. If you're requiring a PowerShell7 be installed then you can also require net8.