CA2008 Do not create tasks without passing a TaskScheduler doesn't appear when pasing method group

#### Analyzer package

[Microsoft.CodeAnalysis.FxCopAnalyzers](https://www.nuget.org/packages/Microsoft.CodeAnalysis.FxCopAnalyzers)

#### Package Version

Example: v2.6.2

#### Diagnostic ID

`CA2008: Do not create tasks without passing a TaskScheduler`
> Do not create tasks unless you are using one of the overloads that takes a TaskScheduler. The default is to schedule on TaskScheduler.Current, which would lead to deadlocks. Either use TaskScheduler.Default to schedule on the thread pool, or explicitly pass TaskScheduler.Current to make your intentions clear.

I could not find a documentation page.

#### Repro steps

Use `TaskFactory.StartNew` as method group parameter.

```
public static class Program
{
public static async Task Main(string[] args)
{
var taskFactory = new TaskFactory();
var actions = new Action[] { Console.WriteLine, Console.WriteLine };
await Task.WhenAll(actions.Select(taskFactory.StartNew)).ConfigureAwait(false);
}
}
```

#### Expected behavior

Waring is displayed.

#### Actual behavior

No warning is displayed.

----------

*I just stumpled upon this "behaivor". It's not important for me I just wanted to report it for documentation reasons.*

Reproduce the CA2008 case using TaskFactory.StartNew as a method-group parameter in the supplied example, then locate the analyzer and its existing CA2008 tests. Confirm the current behavior and identify the test or analyzer entry point that handles method-group calls. Done means the supplied pattern produces the expected warning without changing unrelated diagnostics.

Written by the indexing model from the issue text.