Make DAG construction more permissive (#7460)

Workaround/fix for:

- https://github.com/sigp/lighthouse/issues/7323


  - Remove the `StateSummariesNotContiguousError`. This allows us to continue with DAG construction and pruning, even in the case where the DAG is disjointed. We will treat any disjoint summaries as roots of their own tree, and prune them (as they are not descended from finalized). This should be safe, as canonical summaries should not be disjoint (if they are, then the DB is already corrupt).

beacon_node/beacon_chain/src/migrate.rs

@@ -572,7 +572,8 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> BackgroundMigrator<E, Ho
         if state_summaries_dag_roots.len() > 1 {
             warn!(
                 state_summaries_dag_roots = ?state_summaries_dag_roots,
-                "Prune state summaries dag found more than one root"
+                error = "summaries dag found more than one root",
+                "Notify the devs your hot DB has some inconsistency. Pruning will fix it but devs want to know about it",
             );
         }