[BUGFIX] Cast simulated timestamp to int

`DateTimeImmutable::setTimestamp()` requires the timestamp to be an
integer.

Resolves: #103588
Related: #99627
Related: #98045
Releases: main, 12.4, 11.5
Change-Id: I2dba1b3836063872fa04ea38d58b5bd499a8f7d1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83756


Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>