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/store/Cargo.toml

@@ -25,6 +25,7 @@ redb = { version = "2.1.3", optional = true }
 safe_arith = { workspace = true }
 serde = { workspace = true }
 smallvec = { workspace = true }
+ssz_types = { workspace = true }
 state_processing = { workspace = true }
 strum = { workspace = true }
 superstruct = { workspace = true }