This allows for using new APIs introduced in net6.0.

In order to enable building for net6.0, we also need to revert the thread pool changes in #1939 and #1950. This was already effectively reverted in #1992 by not building for net6.0. Now that we are building for net6.0 again, these if-defs need to be removed.

Questions:

1. Does this need a ReleaseNotes entry? I don't think there is any user-visible affect to this change.
2. Should net5.0 be removed? It has been end-of-life for over a year. I left it in, for now. It can be removed in the future, if we think it should be removed.

cc @NickCraver @mgravell