This is cleanup based on both new .NET functionality (in 6 and 7) as well as a ton of simplifications and things that were missed that were caught due to the cleanup.
This was the last of the major bits of code that was technically distinct from Cleaning. This means that each of the bits of functionality that used to all be in Cleaner are split out into their approrpriate classes.
This change ended up being three-fold:
1. Remove the entirely unused header field filters
2. Move filter running to their appropriate filter classes
3. Separate out DatItem filters into more managable private methods
