Delete RuntimeVariableList::from_vec (#7930)

This method is a footgun because it truncates the list. It is the source of a recent bug:

- https://github.com/sigp/lighthouse/pull/7927


  - Delete uses of `RuntimeVariableList::from_vec` and replace them with `::new` which does validation and can fail.
- Propagate errors where possible, unwrap in tests and use `expect` for obviously-safe uses (in `chain_spec.rs`).

beacon_node/network/src/sync/tests/lookups.rs

@@ -2303,7 +2303,7 @@ mod deneb_only {
                 block,
                 self.unknown_parent_blobs
                     .take()
-                    .map(|vec| RuntimeVariableList::from_vec(vec, max_len)),
+                    .map(|vec| RuntimeVariableList::new(vec, max_len).unwrap()),
             )
             .unwrap();
             self.rig.parent_block_processed(