[TASK] Simplify and unify flash message rendering

* Reverts the change that allowed HTML tags by default in flash messages
* Simplifies flash message markup to be bootstrap alert style
* Delegate flash message rendering completely to module template
* Deprecate render method in flash message
* Minor cleanups in flash message view helper and flash message class

Resolves: #71249
Releases: master
Change-Id: Idbb4bc31372fbc435519496168715e0bcc5cebf2
Reviewed-on: https://review.typo3.org/44592


Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>