Add support for netstandard2.0

[a-jackson]

Requires reverting the code to C# 7.3 and making the DisposeAsync conditional on the net6.0 build

[wparad]

I'm sorry this is too many changes, we can't support the validation that would be required here. As I mentioned in the issue, only if the changes were minimal would we be able to effectively support it. But like this, I'm afraid the burden is too high. In this case, I might suggest forking the repo to make those changes.

[wparad]

After further review, we've agreed internally that we can potentially accept this change with some minor suggested changes. Thank you for providing this!

[wparad]

I've approved this and I'm merging it in. Thank you.