Remove schema migrations for v28 and earlier (#9031)

With LH v8.1.3 supporting Fulu-on-Gnosis, we no longer need these DB migrations. All Lighthouse nodes running in prod will soon be updated to LH v8.0.0+ and schema v28+.

This PR helps with Gloas fork choice changes, by allowing us to avoid updating old schema migrations when adding V29 for Gloas:

- https://github.com/sigp/lighthouse/pull/9025


  


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

beacon_node/beacon_chain/tests/store_tests.rs

@@ -3995,11 +3995,7 @@ async fn schema_downgrade_to_min_version(store_config: StoreConfig, archive: boo
         )
         .await;
 
-    let min_version = if spec.is_fulu_scheduled() {
-        SchemaVersion(27)
-    } else {
-        SchemaVersion(22)
-    };
+    let min_version = CURRENT_SCHEMA_VERSION;
 
     // Save the slot clock so that the new harness doesn't revert in time.
     let slot_clock = harness.chain.slot_clock.clone();