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,
"With the new API it is possible to simultaneously set the body attachments and attachments for the document"