Port/move/create .NET samples

[rolfbjarne]

We're going to sunset the Xamarin samples, so we need to figure out what to do about .NET samples.

We've ported a few of our existing Xamarin samples to .NET in the dotnet branch in the ios-samples repository:

https://github.com/xamarin/ios-samples/commits/dotnet

Tasks:

• Create a new macios-samples repository in the dotnet org. Coordinate with Android so we do don't things too differently.
  • For reference, there's already a dotnet/maui-samples repository.
• Update each .NET sample we have to the latest stable version of .NET (8 at the moment), and move into the new repository.
• Make sure the sample browser on Microsoft's documentation site lists these new samples.

[rolfbjarne]

CC @dalexsoto

[davidbritch]

There are two GitHub actions in maui-samples that you may want to re-use:

• One is manually triggered and builds all the samples in the repo (useful when updating .NET versions).
• One is triggered on PRs, and rebuilds the samples touched by the PR.