Deprecate the DebugWriter class

[jnm2]

Followup from #1944

The class should probably be removed at some point as we don't use it.

I'd like to either do that or remove the Compact Framework comment.

nunit/src/NUnitFramework/nunitlite/DebugWriter.cs

Lines 30 to 35 in 194f38a

	/// <summary>
	/// DebugWriter is a TextWriter that sends its
	/// output to Debug. We don't use Trace because
	/// writing to it is not supported in CF.
	/// </summary>
	public class DebugWriter : TextWriter

[CharliePoole]

I haven't used it for a long time! I used to do it in my nunitlite self-executing tests once in a while. Removing it of course risks breaking someone who has found it and used it.

[stevenaw]

Would deprecating this via the obsolete attribute be desired? If so, this could be a good first issue

[jnm2]

That sounds good to me. @nunit/framework-team?