BatchManagementClient example uses ADAL, which is out of support in December 2022

I've been referring to these samples to figure out how to use the BatchManagementClient. (The other Batch client doesn't support user-assigned managed identity) The package [`Microsoft.IdentityModel.Clients.ActiveDirectory`] is deprecated and will stop receiving updates in December 2022 (3 months as of writing).

https://github.com/Azure-Samples/azure-batch-samples/blob/079a7d24b129bdd21a12efe81bdd54f0c1211aa3/CSharp/AccountManagement/AccountManagement.csproj#L15

The sample should be updated to instead use `Microsoft.Identity.Client`, which would look something like this. It would also be great to update beyond `.net462`.

```csharp
var cred = new DefaultAzureCredential(false); // I'm running this on an azure function, with system managed identity.
var token = cred.GetTokenAsync(new TokenRequestContext(new string[]
{
"https://management.core.windows.net/",
}));
using var batchManagementClient = new BatchManagementClient(new TokenCredentials(token.Result.Token));
```

[`Microsoft.IdentityModel.Clients.ActiveDirectory`]: https://www.nuget.org/packages/Microsoft.IdentityModel.Clients.ActiveDirectory/