[TASK] Modernize code in the Mail system

This is just a general cleanup and tidying to use constructor
promotion, typed properties, and other modern language niceties
in the Mail system. It's mainly to setup for future patches
to reduce the jitter in further refactor work.

Resolves: #96715
Releases: main
Change-Id: I0c05db4d2ad3d43de53342511d9a7c6712f0cf73
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73256


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>