Implement DnnDeprecated source generator for types

[bdukes]

Summary

This PR introduces a source generator into the DNN codebase. This allows the replacement of Obsolete attributes with a new attribute, DnnDeprecated. Usage of the new attribute allows the formatting of the deprecation message to be consistent. It could also, potentially, incorporate a simpler way to link to related documentation, when appropriate.

Implementation Details

This source generator requires that the members which are decorated with the attribute to be marked partial, so that another file can be generated that includes the actual Obsolete attribute. This means that methods and nested types need their enclosing types to also be marked partial.

Drawbacks

Because this source generator relies on members being marked partial, it only applies to members which can be partial. This rules out constructors, finalizers, overloaded operators, property declarations, and event declarations.

It's possible to create a different system which generates these members with the Obsolete attribute, but it will be difficult to find a straightforward approach where the cost is less than the benefit.

[bdukes]

This PR is ready for review.

All Obsolete attributes have been touched, either being turned into DnnDeprecated or being updated to use the template that DnnDeprecated generates.

I also removed a number of Obsolete attributes which were on members where the type was already deprecated.

Since DnnDeprecated requires a version in which the member was deprecated, I tried to find that version where it wasn't already marked. I used 7.0.0 as a placeholder if the history of the deprecation pre-dates the git repo.

[donker]

We discussed this during a technical meeting and agreed this was a great way forward for our management of our API.