[BUGFIX] Remove calls to StringUtility::beginsWith()

The method does not exist in 6.2. Replace with a call to good old
GeneralUtility::isFirstPartOfString().

Resolves: #63937
Releases: 6.2
Change-Id: I3ee6650b062d2bca251db1993f4a52b8ad0048e0
Reviewed-on: http://review.typo3.org/35520
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>