Improves sidekiq delivery method deprecation note

What does this MR do and why?

Adds the proposed change from !122659 (comment 1432119726)

We talked about that administrators should not feel the need to update to webhook and then again migrate to direct email ingestion once released. Because we decided that we will only remove the sidekiq delivery method for mail_room email ingestion, once direct email ingestion has been released in GA.

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