[TASK] Make PHP code of TYPO3 Core PHP 7.4 compliant

This resolves the most common issues related to
PHP 7.4 PHP changes:
- implode() argument order
- using "{}" instead of "[]" in PHP array keys
- PHP notices for invalid array key access

Resolves: #89737
Releases: master, 9.5, 8.7
Change-Id: Iff0b8a46279d328f66ae6bab927aef2bca881774
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62367


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>