Bump the MSRV to 1.62 and using #[derive(Default)] on enums (#3304)

## Issue Addressed

N/A

## Proposed Changes

Since Rust 1.62, we can use `#[derive(Default)]` on enums. ✨ 

https://blog.rust-lang.org/2022/06/30/Rust-1.62.0.html#default-enum-variants

There are no changes to functionality in this PR, just replaced the `Default` trait implementation with `#[derive(Default)]`.

beacon_node/client/src/config.rs

@@ -10,7 +10,7 @@ use types::{Graffiti, PublicKeyBytes};
 const DEFAULT_FREEZER_DB_DIR: &str = "freezer_db";
 
 /// Defines how the client should initialize the `BeaconChain` and other components.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub enum ClientGenesis {
     /// Creates a genesis state as per the 2019 Canada interop specifications.
     Interop {
@@ -21,6 +21,7 @@ pub enum ClientGenesis {
     FromStore,
     /// Connects to an eth1 node and waits until it can create the genesis state from the deposit
     /// contract.
+    #[default]
     DepositContract,
     /// Loads the genesis state from SSZ-encoded `BeaconState` bytes.
     ///
@@ -38,12 +39,6 @@ pub enum ClientGenesis {
     },
 }
 
-impl Default for ClientGenesis {
-    fn default() -> Self {
-        Self::DepositContract
-    }
-}
-
 /// The core configuration of a Lighthouse beacon node.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Config {