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`

changelog/ttsao_p2p-stategen.md

@@ -0,0 +1,3 @@
+### Ignored
+
+- P2p: wire stategen into service for last finalized state and use it for active validator count