[TASK] Harden \TYPO3\CMS\Extbase\Core\Bootstrap

- Use strict type mode
- Use type hints whereever possible

Releases: master
Resolves: #87603
Change-Id: I3430b59e93133d9b19d6520d3fd7cca83c0fdb49
Reviewed-on: https://review.typo3.org/59598


Reviewed-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Tested-by: TYPO3com <noreply@typo3.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>