Final tweaks for v4 (#9)

- Add .NET 9, PostgreSQL 17 support
- Drop .NET 6, PostgreSQL 12 support
- Finalize READMEs

Reviewed-on: #9

src/test_all.sh

@@ -7,8 +7,8 @@ dotnet build BitBadger.Documents.sln --no-restore
 cd ./Tests || exit
 
 export BBDOX_PG_PORT=8301
-PG_VERSIONS=('12' '13' '14' '15' 'latest')
-NET_VERSIONS=('6.0' '8.0')
+PG_VERSIONS=('13' '14' '15' '16' 'latest')
+NET_VERSIONS=('8.0' '9.0')
 
 for PG_VERSION in "${PG_VERSIONS[@]}"
 do