Adding deleted useGPU flag setting back into FTF config
As title describes, the useGPU setting for FastTrackFinder was removed accidently by !61713 (merged). As the FTF config setting is False, and it is False by default in the algorithm code, this does not impact any test results. However, this is setting likely to be changed to take advantage of running the FTF seeding with GPU. Targetting master as this is only for running FTF with ITk geometry.