migrating to net6-7 support (net4.5 and netstandard are deprecated)

[EugeneKrapivin]

all tests seem to pass (inside Github CodeSpaces) for net6 and net7

#25

[EugeneKrapivin]

I would suggest adding DependaBot to keep the few dependencies up to date, and maybe some workflows for PR builds and testing, master branch build, test and publish to nuget. I'd be glad to add them, but it might take some time.