Obsolete the AnsiConsoleFactory class (#585)

`AnsiConsoleFactory` should be an internal static class. Creating an `IAnsiConsole` can be achieved through `Spectre.Console.AnsiConsole.Create(AnsiConsoleSettings)`

Since `AnsiConsoleFactory` is public, it can't be changed from a non static class to a static class so obsoleting it is the second best thing to do.

Eventually, when `AnsiConsoleFactory` becomes internal and static, `AnsiConsole.Create` can be simplified to this (and the private static field `_factory` can be removed)

```
public static IAnsiConsole Create(AnsiConsoleSettings settings)
{
    return AnsiConsoleFactory.Create(settings);
}
```

src/Spectre.Console.Testing/TestConsole.cs

@@ -56,8 +56,7 @@ public sealed class TestConsole : IAnsiConsole, IDisposable
         Input = new TestConsoleInput();
         EmitAnsiSequences = false;
 
-        var factory = new AnsiConsoleFactory();
-        _console = factory.Create(new AnsiConsoleSettings
+        _console = AnsiConsole.Create(new AnsiConsoleSettings
         {
             Ansi = AnsiSupport.Yes,
             ColorSystem = (ColorSystemSupport)ColorSystem.TrueColor,

src/Spectre.Console/AnsiConsole.cs

@@ -5,6 +5,10 @@ namespace Spectre.Console;
 /// </summary>
 public static partial class AnsiConsole
 {
+#pragma warning disable CS0618 // 'AnsiConsoleFactory' is obsolete
+    private static readonly AnsiConsoleFactory _factory = new AnsiConsoleFactory();
+#pragma warning restore CS0618
+
     private static Recorder? _recorder;
     private static Lazy<IAnsiConsole> _console = new Lazy<IAnsiConsole>(
         () =>
@@ -61,8 +65,7 @@ public static partial class AnsiConsole
     /// <returns>An <see cref="IAnsiConsole"/> instance.</returns>
     public static IAnsiConsole Create(AnsiConsoleSettings settings)
     {
-        var factory = new AnsiConsoleFactory();
-        return factory.Create(settings);
+        return _factory.Create(settings);
     }
 
     /// <summary>

src/Spectre.Console/AnsiConsoleFactory.cs

@@ -3,6 +3,7 @@ namespace Spectre.Console;
 /// <summary>
 /// Factory for creating an ANSI console.
 /// </summary>
+[Obsolete("Consider using AnsiConsole.Create instead.")]
 public sealed class AnsiConsoleFactory
 {
     /// <summary>