Final tweaks for v4 (#9)

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

Reviewed-on: #9

src/Directory.Build.props

@@ -1,22 +1,19 @@
 <Project>
   <PropertyGroup>
-    <TargetFrameworks>net6.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <DebugType>embedded</DebugType>
     <GenerateDocumentationFile>false</GenerateDocumentationFile>
     <AssemblyVersion>4.0.0.0</AssemblyVersion>
     <FileVersion>4.0.0.0</FileVersion>
     <VersionPrefix>4.0.0</VersionPrefix>
-    <VersionSuffix>rc5</VersionSuffix>
     <PackageReleaseNotes>From v3.1: (see project site for breaking changes and compatibility)
 - Change ByField to ByFields
 - Support dot-access to nested document fields
 - Add Find*Ordered functions/methods
-
-Release Candidate Changes:
-- from v4-rc4: Field construction functions are now generic.
-- from v4-rc3: Add In/InArray field comparisons, revamp internal comparison handling.
-- from v4-rc2: preserve additional ORDER BY qualifiers.
-- from v4-rc1: add case-insensitive ordering.</PackageReleaseNotes>
+- Add case-insensitive ordering (as of rc2)
+- Preserve additional ORDER BY qualifiers (as of rc3)
+- Add In / InArray comparisons (as of rc4)
+- Field construction functions are generic (as of rc5)</PackageReleaseNotes>
     <Authors>danieljsummers</Authors>
     <Company>Bit Badger Solutions</Company>
     <PackageReadmeFile>README.md</PackageReadmeFile>