[BUGFIX] FormEngine: Do not add <br> to readonly textarea content

Multiline content is displayed in a textarea "as is" by the browser.
There is no need to add <br> tags.

Resolves: #79399
Releases: master, 7.6
Change-Id: Ia628457231a15318dff4fe36f5d3cd3d990d7dc7
Reviewed-on: https://review.typo3.org/51372


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>