Update electra spec references to stable release

[nflaig]

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Before we cut the next release which is likely after consensus-spec release, we should update the spec references to the the stable release instead of -alpha.x.

https://github.com/ethereum/beacon-APIs/blob/4fc68cb7e30973bb948c66084a2b94f1fe57c1c4/types/electra/attestation.yaml#L4