[TASK] Streamline compatibility slots of SignalSlot Dispatcher

The first PSR-14 event received a custom Compatibility slot.
All further "old" slots have been moved to "SlotReplacement" classes,
which is now done with this first PSR-14 event (AfterMailerInitializationEvent)
as well, and moved to a more generic MailerInterface API, now that symfony/mailer
is stable.

Resolves: #90273
Releases: master
Change-Id: I989a941031987733d694335d7e235cf1e2392235
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63098


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>