Rename confidential column in notes tables

As part of Internal Notes: Technical debt cleanup (#363801 - closed) we need to a migration to change column name to internal and do required changes to model validations and error handling.

In this phase, we could override confidential to act as an alias of internal so we do not break services. We should also update our model validations and error handling.

Also to be considered, there is an index on the notes.confidential column:

CREATE INDEX index_notes_on_id_where_confidential ON notes USING btree (id) WHERE (confidential = true);

See #363801 (comment 1025497824) for more context.