Update to .Net6

[ChrisJollyAU]

After a lot of work recently, this seems to be stable enough to merge to the main branch

It has been updated to .Net/EF Core 6 with all the latest dependencies. Most standard things are working with the exception of the normal Jet limitations (Schemas, computed columns, sequences, no skip...take and some basic types that have no equivalent in Jet e.g. DateTimeOffset, TimeSpan, DateOnly, TimeOnly)

Tests: A large number of tests have been updated to make sure they compile and work. Most tests now that do fail in general fall in one of 2 categories

1. Tests that test or include an unsupported feature. Will probably never work
2. Tests that generate a different SQL than what is expected. 2.1. Some of these are because EF Core itself has changed since the test was last looked at and the SQL is by default different 2.2. Others are because of certain functions have to be rewritten for Jet. Most common are the CASE...WHEN, CAST, COALESCE statements which have been rewritten using IIF statements

Will be continuing work on getting more tests working

@lauxjpn or @bubibubi Can we look at getting this merged