[BUGFIX] Use string as constant name in FormProtectionFactory.

Currently the constant itself is used, which leads to PHP notices if it is
not defined yet.

Resolves: #60115
Releases 6.2,6.3
Change-Id: I6932f958a9cb0d01be5910860ebbd78f2c67c58a
Reviewed-on: https://review.typo3.org/31345
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring