Add Heroku to ASP.NET Hosting page

### Is there an existing issue for this?

- [x] I have searched the existing issues

### URL

https://dotnet.microsoft.com/en-us/apps/aspnet/hosting

### Describe what you'd like to see

I was looking for a place to host my .NET application. I found that you can now host .NET on [**heroku**](https://devcenter.heroku.com/categories/dotnet-support)
then I visited https://dotnet.microsoft.com/en-us/apps/aspnet/hosting and I saw that heroku is not present here so I thought this would be a good idea to include heroku in, so people can see that heroku supports .NET

### Additional context

https://help.heroku.com/PAT3YEDU/does-heroku-support-net-applications