fix dynssz annotation for electra.Attestation

[pk910]

Parsing electra blocks with dynssz enabled currently fails for the BeaconBlockBody.Attestations field (size mismatch).

The problem is a wrong annotation in the Attestation container.
dynssz-size shouldn't be set here as there is no ssz-size too.
The intention of that tag was probably to calculate the ssz-max tag dynamically.
The corresponding dynssz tag is called dynssz-max.

[pk910]

@mcdee
ready for review :)