2.1 Update config for 2.x Azure KV config provider topic samples

[guardrex]

Based on the updates to config for 2.1 adding chained config, we can move the 2.x Azure Key Vault config provider topic samples (sample 1, sample 2) to a nicer config setup ...

WebHost.CreateDefaultBuilder(args)
    .ConfigureAppConfiguration((context, config) =>
    {
        var builtConfig = config.Build();

        var keyVaultConfigBuilder = new ConfigurationBuilder();

        keyVaultConfigBuilder.AddAzureKeyVault(
           $"https://{builtConfig["Vault"]}.vault.azure.net/",
           builtConfig["ClientId"],
           builtConfig["ClientSecret"];

        config.AddConfiguration(keyVaultConfigBuilder);
   })
   .Build()

@scottaddie Can you create a label for 2.1 for this? ... or mark it blocked right now?

[scottaddie]

@guardrex Label created and applied

[Rick-Anderson]

Make sure this is updated to the latest version without tabs from previous versions. If you need previous versions, point to https://github.com/aspnet/Docs/blob/master/aspnetcore/common/_static/9-25-17.pdf

This is revised: Keep the 1.x samp, update the 2.0 samp to 2.1.