Switch to NOT_SET, deprecate NotSet

[braingram]

As mentioned #2091 (review) there is a viable option for switching to NOT_SET and deprecating NotSet.

It would be good to spread this out over releases to allow downstream to adapt with:

• an introduction of NOT_SET, switch internal uses from NotSet to NOT_SET, add tests to confirm NotSet continues to work, make a release with these changes
• in a minor version (likely in 6.x) deprecate NotSet, allowing downstream an opportunity to switch to NOT_SET without complicated per-asdf-version logic
• in the following major version, remove NotSet