[BUGFIX] Avoid PHP deprecation in TextElement readOnly mode

This has already been fixed upstream as a drive-by change.

Resolves: #102388
Related: #97433
Releases: 11.5
Change-Id: Id3f8f541b077d4d3b51bdd794847249b4229768f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81760


Reviewed-by: Thomas Hohn <tho@gyldendal.dk>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>