[BUGFIX] Fix letter only HTML5 regex

The regular expression for validation letter only strings
now allows any number of characters.

Resolves: #58966
Releases: 2.1
Change-Id: I847c05b91a5a12fe2ba6e0c1cec24e86ff3cbf77
Reviewed-on: https://review.typo3.org/30236
Reviewed-by: Alex Kellner
Tested-by: Alex Kellner