[BUGFIX] Tighten CleanerTaskTest setup for changed PHP 8.1 behaviour

CleanerTaskTest dynamically calculates the needed 'period' time to use
for testing, if one page records will survive the cleaning task, as
not being outside this periode grace time. This has worked quite some
time, but may crash now in a quite narrow timeframe, which failed core
ci tests, if they were run in this timeframe - and made it reproducible
only for a night owl, as the timeframe lays between UTC '00:00-01:00'.

PHP8.1.0RC6 included a regression bugfix which introduced another bug,
which leads to inconsinstent calculation behaviour for using 'diff()'
method of '\DateTime' or '\DateTimeImmutable', if timezone switches are
needed and there are the first hour leap (or two hour in summer time).
This is already fixed in PHP8.1.0 release.

As we do not really need that 1 hour leap calculation here, we are able
to tighten the test setup here and calculate the needed dynamic period
in way to work around this bug for PHP8.1.0RC6, be backwards compatible
and also being safe for 8.1.0 release.

PHP regression fix introduced new bug (8.1.0RC6):
https://github.com/php/php-src/commit/904933e9185664148ce54

PHP bugfix released with 8.1.0:
https://github.com/php/php-src/commit/e4679ef0c2d5a0a81cb6e

This patch uses '00:00:00' for the time part of both dates and further
uses 'UTC' timezone enforcement to calculate the period time, which
stabilizes this testcase and lets night owls do their stuff undisturbed.

Resolves: #96128
Releases: master, 11.5
Change-Id: Ib1f4201ce8556ae8f22eeb4463732c62394e3c14
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72346


Reviewed-by: Stephan Großberndt <stephan.grossberndt@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>