Fix Clippy for Rust 1.90 beta (#7826)

Fix Clippy for recently released Rust 1.90 beta. There may be more changes required when Rust 1.89 stable is released in a few days, but possibly not 🤞

crypto/bls/tests/tests.rs

@@ -370,7 +370,7 @@ macro_rules! test_suite {
         }
 
         impl OwnedSignatureSet {
-            pub fn multiple_pubkeys(&self) -> SignatureSet {
+            pub fn multiple_pubkeys(&self) -> SignatureSet<'_> {
                 let signing_keys = self.signing_keys.iter().map(Cow::Borrowed).collect();
                 SignatureSet::multiple_pubkeys(&self.signature, signing_keys, self.message)
             }

crypto/eth2_keystore/src/json_keystore/hex_bytes.rs

@@ -37,7 +37,7 @@ impl TryFrom<String> for HexBytes {
     fn try_from(s: String) -> Result<Self, Self::Error> {
         // Left-pad with a zero if there is not an even number of hex digits to ensure
         // `hex::decode` doesn't return an error.
-        let s = if s.len() % 2 != 0 {
+        let s = if !s.len().is_multiple_of(2) {
             format!("0{}", s)
         } else {
             s