[BUGFIX] Use correct implode(string, array) syntax

The usage of implode(array, string) has been marked as
deprecated by PHP and triggers PHP Deprecation warnings
by using PHP 7.4.

Resolves: #89987
Releases: master, 9.5
Change-Id: I4094744bd067203856dc305bb7e1651797d0c959
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62741


Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Markus Klösges <mkloesges@gmx.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>