Upgrade rand_core (#2201)

## Issue Addressed

N/A

## Proposed Changes

Upgrade `rand_core` to latest version to fix https://rustsec.org/advisories/RUSTSEC-2021-0023

beacon_node/beacon_chain/Cargo.toml

@@ -46,7 +46,7 @@ futures = "0.3.7"
 genesis = { path = "../genesis" }
 integer-sqrt = "0.1.5"
 rand = "0.7.3"
-rand_core = "0.5.1"
+rand_core = "0.6.2"
 proto_array = { path = "../../consensus/proto_array" }
 lru = "0.6.0"
 tempfile = "3.1.0"

beacon_node/beacon_chain/src/test_utils.rs

@@ -15,7 +15,7 @@ use genesis::interop_genesis_state;
 use parking_lot::Mutex;
 use rand::rngs::StdRng;
 use rand::Rng;
-use rand_core::SeedableRng;
+use rand::SeedableRng;
 use rayon::prelude::*;
 use slog::Logger;
 use slot_clock::TestingSlotClock;