[TASK] Move "pageUnavailable_force" into a PSR-15 middleware

The check if the frontend is in "maintenance mode",
set by $GLOBALS['TYPO3_CONF_VARS']['FE']['pageUnavailable_force'],
is moved into a custom PSR-15 based middleware,
effectively decoupling this logic from TSFE object.

Resolves: #83917
Releases: master
Change-Id: I38c42069b82ca2403df9aa4b0e66410a3dfe6404
Reviewed-on: https://review.typo3.org/55721


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>