[BUGFIX] Input and save date properly when USdateFormat is set

Currently, the value of `USdateFormat` is disregarded because the
corresponding property, `FormEngineValidation.USmode`, is set to zero
regardless in `FormEngineValidation.initialize()`. This change will ensure
that `USmode` will remain 1 without conflicting with core functionality
that relies on `USmode` being set *before* `initialize()` is called.

Resolves: #86679
Releases: master, 10.4
Related-to: #91105
Change-Id: I5584ca7fe92742e28147f9883fedf53e0556f1b1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72034


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>