[BUGFIX] Render dbType=date/datetime readonly fields properly

When a TCA field of dbType=date / dateTime is rendered, and the field
is marked as read-only, the value is not evaluated, as it never ever modified
as UNIX timestamp.

Resolves: #88450
Resolves: #90413
Releases: master, 9.5
Change-Id: I29c1acda07f11729d84668f51efdfeff4a4d99c5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63626


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>