Bump Microsoft.AspNetCore.App in /test/Tests.Integration

Bumps [Microsoft.AspNetCore.App](https://github.com/aspnet/AspNetCore) from 2.1.5 to 2.1.15.
- [Release notes](https://github.com/aspnet/AspNetCore/releases)
- [Changelog](https://github.com/dotnet/aspnetcore/blob/main/docs/ReleasePlanning.md)
- [Commits](dotnet/aspnetcore@2.1.5...v2.1.15)

---
updated-dependencies:
- dependency-name: Microsoft.AspNetCore.App
  dependency-type: direct:production
...

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

test/Tests.Integration/Tests.Integration.csproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.App" Version="2.1.5" />
+    <PackageReference Include="Microsoft.AspNetCore.App" Version="2.1.15" />
     <PackageReference Include="Microsoft.AspNetCore.Authentication.OAuth" Version="2.1.2" />
     <PackageReference Include="Microsoft.AspNetCore.Http.Abstractions" Version="2.1.1" />
     <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="2.1.3" />