[BUGFIX] Re-add field explanation text for form finisher checkboxes

This patch re-adds support for the `fieldExplanationText` property of
form finisher checkboxes. It was removed with #95086, but is actually
still used in some finishers, e.g. email finisher. Said patch also
introduced wrong HTML for the checkbox label, which is fixed as well.

Resolves: #101831
Related: #95086
Releases: main, 12.4
Change-Id: Ia1fe626ee5375f6d13adad8274b04be28345ed65
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80843


Tested-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>