35676 remove deprecated apis

- Remove deprecated APIs from CDTDatastore and associated tests from code base.
- Remove usage of deprecated APIs from todo sample.
- Rename description property in CDTTodo  to taskDescription, stops it conflicting with NSObject description 
- Set ivars directly in CDTTodo init method
- Add an API migration document to assist users in migrating to the new API

The biggest area to focus on for this review is the addition of the API migration doc,
I guess this ought to have a call to createDocumentForRevision at the end and a note saying that process is similar when calling updateDocumentForRevision (we don't need tonnes of detail as the CRUD document goes through all of the possibilities)