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)]`.

crypto/eth2_keystore/src/json_keystore/kdf_module.rs

@@ -58,9 +58,10 @@ impl Kdf {
 }
 
 /// PRF for use in `pbkdf2`.
-#[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
+#[derive(Debug, PartialEq, Clone, Serialize, Deserialize, Default)]
 pub enum Prf {
     #[serde(rename = "hmac-sha256")]
+    #[default]
     HmacSha256,
 }
 
@@ -73,12 +74,6 @@ impl Prf {
     }
 }
 
-impl Default for Prf {
-    fn default() -> Self {
-        Prf::HmacSha256
-    }
-}
-
 /// Parameters for `pbkdf2` key derivation.
 #[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
 #[serde(deny_unknown_fields)]