[TASK] Move BE-related bootstrap code to PSR-15 middleware

Various functionality used by both backend request handlers is moved
into PSR-15 compatible middleware, making the following Bootstrap
methods obsolete:

- checkLockedBackendAndRedirectOrDie()
- checkBackendIpOrDie()
- checkSslBackendAndRedirectIfNeeded()
- initializeBackendTemplate()
- initializeOutputCompression()
- sendHttpHeaders()

The following methods are still needed in other contexts however
- initializeBackendRouter()
- loadExtTables()
- endOutputBufferingAndCleanPreviousOutput()
- initializeBackendAuthentication()
- initializeBackendUser()
- initializeLanguageObject()

Resolves: #83792
Releases: master
Change-Id: I6772bbb73f17100dfc5116f58f1202f52fee1e8d
Reviewed-on: https://review.typo3.org/55538


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