Deprecate option to update notes confidentiality via REST and GraphQL APIs

Currently, it is possible to update notes confidentiality via GraphQL (see UpdateNote) and REST (see #update_note) endpoints, but it should be reverted.

Once a discussion is marked as confidential it should not be switched back to non-confidential (and vice-versa).

As the feature is behind a feature flag this should not be considered a breaking change. However, we can make it clear in documentation that this API endpoint has been deprecated for a period before removing the attribute entirely from the APIs.

Proposal

• Deprecate :confidential argument in public endpoints

Tasks

• Deprecate the GraphQL endpoint(s) (Guidance)
• Deprecate the REST API endpoint(s)
• Create a follow up issue to remove mutable note confidentiality from APIs entirely and schedule for appropriate milestone #358734