[TASK] Use PSR-15 RequestHandler in Install Tool

Install Tool still uses TYPO3's own RequestHandler interface which
exists since TYPO3 v7, but is superseded with PSR-15's
RequestHandler concept.

Install Tool now uses PSR-15 and the old interface in order
to still use the priority concept.

In a further step, both RequestHandlers should be migrated to
Middleware+RequestHandler concept.

Resolves: #86451
Releases: master
Change-Id: I55cf31a2b803951491ba072969a4cff68ef2c2ac
Reviewed-on: https://review.typo3.org/58469


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>