Fix voluntary exit signatures

[0xfornax]

Fix the exit signatures as they were failing after the Dencun upgrade on Holesky.
Always using the CAPELLA_FORK_VERSION when computing the domain for the voluntary exit signature.
Domain() function was deprecated so it was replaced by ComputeDomain()

[jshufro]

will this work on mainnet? what happens when the CAPELLA_FORK_VERSION field is null, eg on a pre-capella testnet?

[0xfornax]

will this work on mainnet? what happens when the CAPELLA_FORK_VERSION field is null, eg on a pre-capella testnet?

Yes, on mainnet the signature uses the current fork version (which is also Capella).
I don't think it's worth trying to support past forks and also don't see the benefits of running such forks on testnets as they don't allow us to replicate the production environment.