Stop publishing the Microsoft.NET.Build.Containers package when the SDK is released

### Is your feature request related to a problem? Please describe.
Now that the containers package is fully integrated into the SDK, there's no reason for the package to be published anymore. In fact, it can lead to end-user errors when they use e.g. an 8.0 SDK and update to the 9.0 version of the package - because the tasks target net9.0 and there's no information for NuGet to see that would _stop_ the package from working on .NET 8, NuGet happily lets users update.

### Describe the solution you'd like
We should stop publishing the package, but continue embedding it into the .NET SDK.
The existing check to warn users to not use the package should be kept.