Deprecating fields in CDEvents

[afrittoli]

We would like to identify and document the process for deprecating fields in CDEvents.

We discussed this during a working group and identified the following options:

1. Document in the markdown, and define an ETA for removal
2. Keep the deprecated fields forever
3. Just update the version and drop the field

Related PR: PR not urgent, good to use to define the deprecation process

The general agreement was for option (1). JSONSChema allows for deprecations and so we could generate the markdown for deprecations automatically.