[BUGFIX] Show validation errors for fields with legend in form engine

Since #97330, a form engine elements label
might not be represented by a <label> tag,
but by using a <legend> tag.

This patch now ensures that those elements
are also correctly considered within form
engine validation.

Resolves: #101221
Related: #97330
Releases: main, 12.4
Change-Id: Iac4ef9b198bd7a7b8c75e53b83e45117be348c98
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79636


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