`ECDSA`: Use hexadecimal literals (#4317)

contracts/utils/cryptography/ECDSA.sol

@@ -197,7 +197,7 @@ library ECDSA {
         /// @solidity memory-safe-assembly
         assembly {
             let ptr := mload(0x40)
-            mstore(ptr, "\x19\x01")
+            mstore(ptr, hex"19_01")
             mstore(add(ptr, 0x02), domainSeparator)
             mstore(add(ptr, 0x22), structHash)
             data := keccak256(ptr, 0x42)
@@ -211,6 +211,6 @@ library ECDSA {
      * See {recover}.
      */
     function toDataWithIntendedValidatorHash(address validator, bytes memory data) internal pure returns (bytes32) {
-        return keccak256(abi.encodePacked("\x19\x00", validator, data));
+        return keccak256(abi.encodePacked(hex"19_00", validator, data));
     }
 }