!per #15428 Deprecate deleteMessage
!per #15427 Deprecate deleteMessage and writeConfirmations in journal plugin api
!per Additional deprecations and cleanup
!per Change for journal plugin compatibility

* A few more adjustments, found when verifying source compatibility of a few
  journal plugins
* Note that PersistentId will be removed with deleteMessage and we should
  not break plugins because of that
* Add missing section of at-least-once delivery in migration guide
Merge pull request #15462 from akka/wip-15427-deprecate-delete-and-confirm-patriknw

Deprecate deleteMessage, writeConfirmations and some more