Bump Microsoft.AspNetCore.Mvc.Testing from 6.0.11 to 8.0.20 (#35)

---
updated-dependencies:
- dependency-name: Microsoft.AspNetCore.Mvc.Testing
  dependency-version: 8.0.20
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

test/Dfe.Analytics.AspNetCore.Tests/Dfe.Analytics.AspNetCore.Tests.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="6.0.11" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="8.0.20" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
     <PackageReference Include="Moq" Version="4.18.2" />
     <PackageReference Include="xunit" Version="2.4.1" />