Downgrade ConfigurationManager from 6.0.1 to 6.0.0

fluentassertions#2673 bumped from 4.4.0 -> 6.0.1, but 6.0.0 should be enough.
As a testing nuget we should pick the lowest possible version of a package to avoid that tests use a newer package than what is used in production.

Author: jnyrup

Src/FluentAssertions/FluentAssertions.csproj

@@ -72,7 +72,7 @@
         <Compile Remove="SystemExtensions.cs" />
       </ItemGroup>
       <ItemGroup>
-        <PackageReference Include="System.Configuration.ConfigurationManager" Version="6.0.1" />
+        <PackageReference Include="System.Configuration.ConfigurationManager" Version="6.0.0" />
       </ItemGroup>
     </When>
     <When Condition="'$(TargetFramework)' == 'netstandard2.1'">