* context: #1716https://github.com/DapperLib/Dapper/issues/1716
- change type-map to allow nullable
- change DateTime/TimeSpan to null
- do not specify DbType for null
- semi-breaking change: swap GetDbType to SetDbType, noting that it is marked [Obsolete] and internal-use-only
- semi-breaking change: make LookupDbType return nullable (same internal-use/[Obsolete])
* add test for Npgsql 6.0.0-rc.1
* try list parameters (like arrays)
* never assign -1 (sentinel) as the .DbType
* postgres tests
* initial net5.0 tweaks; also fix current IDE warnings
* lib updates and fixes
* more cleanup
* fix snafu
* skip locals init
* disable xunit shadow copy in tests
* test records
* explore scenario that currently works re #1571