Final tweaks for v4 (#9)

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

Reviewed-on: #9

src/Postgres/README.md

@@ -5,11 +5,16 @@ This package provides a lightweight document library backed by [PostgreSQL](http
 ## Features
 
 - Select, insert, update, save (upsert), delete, count, and check existence of documents, and create tables and indexes for these documents
+- Automatically generate IDs for documents (numeric IDs, GUIDs, or random strings)
 - Address documents via ID, via comparison on any field, via equality on any property (using JSON containment, on a likely indexed field), or via condition on any property (using JSON Path queries)
 - Access documents as your domain models (<abbr title="Plain Old CLR Objects">POCO</abbr>s)
 - Use `Task`-based async for all data access functions
 - Use building blocks for more complex queries
 
+## Upgrading from v3
+
+There is a breaking API change for `ByField` (C#) / `byField` (F#), along with a compatibility namespace that can mitigate the impact of these changes. See [the migration guide](https://bitbadger.solutions/open-source/relational-documents/upgrade-from-v3-to-v4.html) for full details.
+
 ## Getting Started
 
 Once the package is installed, the library needs a data source. Construct an `NpgsqlDataSource` instance, and provide it to the library: