[BUGFIX] Make user avatar buttons accessible via keyboard

Both "clear" and "add" buttons for the backend user avatar selection
are currently not accessible via keyboard. In order to solve this
accessibility issue, both buttons have been converted from links to
html buttons to trigger the onclick handler when using the keyboard.

Resolves: #90293
Releases: master, 9.5
Change-Id: I263a282a4bee33abdb88f9850d230a453365b7ab
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63148


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Michael Telgkamp <michael.telgkamp@mindscreen.de>
Tested-by: Torben Hansen <derhansen@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>