[BUGFIX] Fix displaying issue of buttons inside form fields

By setting the display property of buttons inside of input-groups to
"flex", the height of the element inside the button can be easier
controlled.

Resolves: #95451
Releases: master, 11.5
Change-Id: Ia41d6bf4421d8ad078509893c8b0177708767e5c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71459


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>