Fix GNNHS VertexDecoratorAlg failure (ATLASRECTS-8178)

Following !78633 (merged), the sumPt vertex decoration can get properly locked by the PhotonVertexSelection, creating a crash when running the VertexDecoratorAlg which attempts to modify it.

In practice, this was already fixed in main with !77093 (merged)

Sweeping therefore some relevant commits from main to 24.0 to avoid the failure of the GNNHS unit test:

• !77025 (merged)
• !77093 (merged)

Removes also some unnecessary SG::AuxElement::Decorator

The commits should not be squashed to preserve history and facilitate the sweep in main

Closes ATLASRECTS-8178

FYI @jburzyns @gfrattar @ssnyder