chore: update Testably.Abstractions to v9.0.0 (#740)

- Use v9.0.0 as package reference to Testably.Abstractions
- Update badge in README to reference the Testing package

Author: vbreuss

Directory.Packages.props

@@ -48,7 +48,7 @@
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0"/>
   </ItemGroup>
   <ItemGroup>
-    <PackageVersion Include="Testably.Abstractions" Version="9.0.0-pre.2"/>
+    <PackageVersion Include="Testably.Abstractions" Version="9.0.0"/>
     <PackageVersion Include="Testably.Abstractions.Testing" Version="3.3.0-pre.1"/>
   </ItemGroup>
   <ItemGroup>

README.md

@@ -1,5 +1,5 @@
 ![Testably.Abstractions](https://raw.githubusercontent.com/Testably/Testably.Abstractions/main/Docs/Images/social-preview.png)  
-[![Nuget](https://img.shields.io/nuget/v/Testably.Abstractions)](https://www.nuget.org/packages/Testably.Abstractions) 
+[![Nuget](https://img.shields.io/nuget/v/Testably.Abstractions.Testing)](https://www.nuget.org/packages/Testably.Abstractions.Testing) 
 [![Build](https://github.com/Testably/Testably.Abstractions/actions/workflows/build.yml/badge.svg)](https://github.com/Testably/Testably.Abstractions/actions/workflows/build.yml) 
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=Testably_Testably.Abstractions&metric=alert_status)](https://sonarcloud.io/summary/new_code?id=Testably_Testably.Abstractions) 
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=Testably_Testably.Abstractions&metric=coverage)](https://sonarcloud.io/summary/new_code?id=Testably_Testably.Abstractions)