Update to match Solidity pragma (#5198)

Signed-off-by: Hadrien Croubois <hadrien.croubois@gmail.com>

contracts/utils/cryptography/Hashes.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity ^0.8.20;
 
 /**
  * @dev Library of standard hash functions.

contracts/utils/structs/MerkleTree.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.8.0;
+pragma solidity ^0.8.20;
 
 import {Hashes} from "../cryptography/Hashes.sol";
 import {Arrays} from "../Arrays.sol";