p2p: wire stategen into service for last finalized state (#16034)

This PR removes the last production usage for: `LastArchivedRoot` by

- extend the P2P config to accept a `StateGen` dependency and wire it up
from the beacon node
- update gossip scoring to read the active validator count via stategen
using last finalized block root

note: i think the correct implementation should process last finalizes
state to current slot, but that's a bigger change i dont want to make in
this PR, i just want to remove usages for `LastArchivedRoot`

beacon-chain/node/node.go

@@ -663,6 +663,7 @@ func (b *BeaconNode) registerP2P(cliCtx *cli.Context) error {
 		EnableUPnP:            cliCtx.Bool(cmd.EnableUPnPFlag.Name),
 		StateNotifier:         b,
 		DB:                    b.db,
+		StateGen:              b.stateGen,
 		ClockWaiter:           b.clockWaiter,
 	})
 	if err != nil {