This commit removes the 'prev_rendered_content_version'
field from:
* the 'edit_history' object within message objects in the
API response of `GET /messages`, `GET /messages/{message_id}`
and `POST /zulip-outgoing-webhook`.
* the 'update_message' event type
as it is an internal server implementation detail not used
by any client.
Note: The field is still stored in the 'edit_history' column
of the 'Message' table as it will be helpful when making
major changes to the markup rendering process.
656c65c74c