Update attestation packing for Deneb

[michaelsproul]

Description

Currently the lengthening of attestation validity in Deneb is not being taken advantage of in the op pool. We still filter attestations by the last 32 slots here:

lighthouse/beacon_node/operation_pool/src/lib.rs

Lines 232 to 235 in 8660043

	.filter(|att| {
	att.data.slot + spec.min_attestation_inclusion_delay <= state.slot()
	&& state.slot() <= att.data.slot + T::slots_per_epoch()
	})

Post-Deneb, we should be keeping any attestation from the current or previous epoch.

Steps to resolve

I propose we resolve this after merging Deneb to unstable, so as not to block it further. It's really just an optimisation to block packing to take advantage of the new Deneb rules. At the same time, block packing is being overhauled in #4507, so it might make sense to get that PR in before making this change.

[realbigsean]

I'm gonna mark this blocked on #4507 for now, feel free to drop that tag though if we want it in sooner