Stop using version macros in sample projects

I am seeing [this pattern](https://github.com/dotnet/machinelearning-samples/blob/master/samples/csharp/getting-started/Ranking_Web/WebRanking/WebRanking.csproj#L9) in sample projects. I consider it an anti-pattern. You cannot just copy these samples and use them. We don't use this pattern elsewhere.

```csharp




```

The macros are defined in the root of the samples directory in [Directory.Build.props](https://github.com/dotnet/machinelearning-samples/blob/master/samples/Directory.Build.props).

I also found examples where it is [done wrong](https://github.com/dotnet/machinelearning-samples/blob/master/samples/csharp/getting-started/Regression_AutoML/TaxiFarePrediction/TaxiFarePrediction.csproj#L14):

```csharp

```

I made a [PR to add dependabot support for this project](https://github.com/dotnet/machinelearning-samples/pull/621). The dependabot model is a much better user experience than using these macros. The downside is that it will create a lot of PRs that need to be managed, but that's just a function of the number of samples provided.