Bump ssz_types to v0.12.2 (#8032)

https://github.com/sigp/lighthouse/issues/8012


  Replace all instances of `VariableList::from` and `FixedVector::from` to their `try_from` variants.

While I tried to use proper error handling in most cases, there were certain situations where adding an `expect` for situations where `try_from` can trivially never fail avoided adding a lot of extra complexity.


Co-Authored-By: Mac L <mjladson@pm.me>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/network/src/network_beacon_processor/gossip_methods.rs

@@ -2742,6 +2742,20 @@ impl<T: BeaconChainTypes> NetworkBeaconProcessor<T> {
                     }
                 }
             }
+            AttnError::SszTypesError(_) => {
+                error!(
+                    %peer_id,
+                    block = ?beacon_block_root,
+                    ?attestation_type,
+                    "Rejecting attestation due to a critical SSZ types error"
+                );
+                self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Reject);
+                self.gossip_penalize_peer(
+                    peer_id,
+                    PeerAction::MidToleranceError,
+                    "attn_ssz_types_error",
+                );
+            }
         }
 
         debug!(