Fix WriteDecorHandleKey type for ftag electron barcode decorators

Clone of !64193 (closed)

See type mismatch:

• https://gitlab.cern.ch/atlas/athena/-/blob/main/PhysicsAnalysis/JetTagging/FlavorTagDiscriminants/src/SoftElectronTruthDecoratorAlg.cxx#L65
• https://gitlab.cern.ch/atlas/athena/-/blob/main/PhysicsAnalysis/JetTagging/FlavorTagDiscriminants/FlavorTagDiscriminants/SoftElectronTruthDecoratorAlg.h#L48-50

Thanks @dguest

Ping @dkobylia