[BUGFIX] Make RTE validate required

Currently, the RTE did not validate when required=true
was set.

This has been changed to update the source on change
which triggers validation again.

Resolves: #100838
Releases: main, 12.4
Change-Id: I3bc9a9c43d23d8334f3f5694ef52f626d12c424c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79776


Tested-by: Andreas Nedbal <andy@pixelde.su>
Reviewed-by: Andreas Nedbal <andy@pixelde.su>
Tested-by: core-ci <typo3@b13.com>