Use altair spec objects in lightclient fork upgrade

[VolodymyrBg]

• Replace pre types from bellatrix.LightClient* to altair.LightClient* in specs/capella/light-client/fork.md.
• In upgrade_lc_header_to_capella, explicitly set execution=ExecutionPayloadHeader() and execution_branch=ExecutionBranch() for pre-Capella data.

why:

• The Capella fork upgrades light-client data from Altair, not Bellatrix. The file itself states the upgrade path is Altair → Capella, and networking tables confirm that through Bellatrix the wire types remain altair.. The repo contains no bellatrix.LightClient definitions.
• Capella adds execution and execution_branch to LightClientHeader. For slots before CAPELLA_FORK_EPOCH, validation requires these fields to be zeroed. Without initializing them, upgraded pre-fork headers would be invalid per is_valid_light_client_header.

[jtraglia]

LGTM, I agree we should be using the altair spec object here.