Bump Microsoft.AspNetCore.Mvc.Testing from 7.0.13 to 7.0.20 in /src

Bumps [Microsoft.AspNetCore.Mvc.Testing](https://github.com/dotnet/aspnetcore) from 7.0.13 to 7.0.20.
- [Release notes](https://github.com/dotnet/aspnetcore/releases)
- [Changelog](https://github.com/dotnet/aspnetcore/blob/main/docs/ReleasePlanning.md)
- [Commits](dotnet/aspnetcore@v7.0.13...v7.0.20)

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

Signed-off-by: dependabot[bot] <support@github.com>

src/AzureEventGridSimulator.Tests/AzureEventGridSimulator.Tests.csproj

@@ -8,7 +8,7 @@
 
   <ItemGroup>
     <PackageReference Include="Azure.Messaging.EventGrid" Version="4.21.0" />
-    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="7.0.13" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="7.0.20" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
     <PackageReference Include="Shouldly" Version="4.2.1" />