Update still uses `client_state.latest_slot` rather than `header.trusted_slot`

[srdtrk]

Summary of Bug

With the addition of historical updates in #503, verify client now uses header.trusted_slot to pick the consensus state to use for verify the header itself. However, update_client still uses client_state.latest_slot.

We can also instead add the requirement that latest_slot is in the same sync committee period trusted_slot, in which case, it wouldn't matter which one we used.

Version

main

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[linear]

IBCSOL-261 Update still uses `client_state.latest_slot` rather than `header.trusted_slot`