Moves scheduled removal for sidekiq delivery method to 18.0

What does this MR do and why?

Deprecation issue: Deprecate sidekiq delivery method for email ing... (#398132)

Moves scheduled removal for sidekiq delivery method to 18.0

The original removal milestone was 17.0 but we don't have the capacity to remove it in this milestone. Additionally we promised to deliver a proper replacement and only remove it once it's been enabled. Right now only the webhook delivery method exists which is not easy to set up for multi node setups using Omnisbus.

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