Bump Shouldly from 4.0.0-beta0004 to 4.0.0 (graphql-dotnet#1999)

Bumps [Shouldly](https://github.com/shouldly/shouldly) from 4.0.0-beta0004 to 4.0.0.
- [Release notes](https://github.com/shouldly/shouldly/releases)
- [Changelog](https://github.com/shouldly/shouldly/blob/master/BREAKING%20CHANGES.txt)
- [Commits](shouldly/shouldly@v4.0.0-beta0004...v4.0.0)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/Tests.props

@@ -6,7 +6,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
-    <PackageReference Include="Shouldly" Version="4.0.0-beta0004" />
+    <PackageReference Include="Shouldly" Version="4.0.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
   </ItemGroup>