downgrade contracts to be compatible with solidity 0.8.15

contracts/PreConfirmations.sol

@@ -1,12 +1,12 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.20;
+pragma solidity ^0.8.15;
 
 import {ECDSA} from "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
 import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
 
 import {IProviderRegistry} from "./interfaces/IProviderRegistry.sol";
 import {IUserRegistry} from "./interfaces/IUserRegistry.sol";
-import {MessageHashUtils} from "@openzeppelin/contracts/utils/cryptography/MessageHashUtils.sol";
+import {ECDSA} from "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
 
 /**
  * @title PreConfCommitmentStore - A contract for managing preconfirmation commitments and bids.
@@ -110,7 +110,7 @@ contract PreConfCommitmentStore is Ownable {
         address _providerRegistry,
         address _userRegistry,
         address _oracle
-    ) Ownable(msg.sender) {
+    ) {
         oracle = _oracle;
         providerRegistry = IProviderRegistry(_providerRegistry);
         userRegistry = IUserRegistry(_userRegistry);
@@ -146,7 +146,7 @@ contract PreConfCommitmentStore is Ownable {
         uint64 _blockNumber
     ) public view returns (bytes32) {
         return
-            MessageHashUtils.toTypedDataHash(
+            ECDSA.toTypedDataHash(
                 DOMAIN_SEPARATOR_BID,
                 keccak256(
                     abi.encode(
@@ -175,7 +175,7 @@ contract PreConfCommitmentStore is Ownable {
         string memory _bidSignature
     ) public view returns (bytes32) {
         return
-            MessageHashUtils.toTypedDataHash(
+            ECDSA.toTypedDataHash(
                 DOMAIN_SEPARATOR_PRECONF,
                 keccak256(
                     abi.encode(

contracts/ProviderRegistry.sol

@@ -1,8 +1,8 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.20;
+pragma solidity ^0.8.15;
 
 import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
-import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
+import {ReentrancyGuard} from "@openzeppelin/contracts/security/ReentrancyGuard.sol";
 import {PreConfCommitmentStore} from "./PreConfirmations.sol";
 import {IProviderRegistry} from "./interfaces/IProviderRegistry.sol";
 
@@ -75,7 +75,7 @@ contract ProviderRegistry is IProviderRegistry, Ownable, ReentrancyGuard {
         uint256 _minStake,
         address _feeRecipient,
         uint16 _feePercent
-    ) Ownable(msg.sender) {
+    ) {
         minStake = _minStake;
         feeRecipient = _feeRecipient;
         feePercent = _feePercent;

contracts/UserRegistry.sol

@@ -1,8 +1,8 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.20;
+pragma solidity ^0.8.15;
 
 import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
-import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
+import {ReentrancyGuard} from "@openzeppelin/contracts/security/ReentrancyGuard.sol";
 import {IUserRegistry} from "./interfaces/IUserRegistry.sol";
 
 /// @title User Registry
@@ -71,7 +71,7 @@ contract UserRegistry is IUserRegistry, Ownable, ReentrancyGuard {
         uint256 _minStake,
         address _feeRecipient,
         uint16 _feePercent
-    ) Ownable(msg.sender) {
+    ) {
         minStake = _minStake;
         feeRecipient = _feeRecipient;
         feePercent = _feePercent;

contracts/interfaces/IProviderRegistry.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.20;
+pragma solidity ^0.8.15;
 
 interface IProviderRegistry {
     function registerAndStake() external payable;

contracts/interfaces/IUserRegistry.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.20;
+pragma solidity ^0.8.15;
 
 interface IUserRegistry {
     struct PreConfCommitment {

hardhat.config.js

@@ -4,7 +4,7 @@ require("solidity-docgen");
 
 /** @type import('hardhat/config').HardhatUserConfig */
 module.exports = {
-  solidity: "0.8.20",
+  solidity: "0.8.15",
   networks: {
     localhost: {
       url: "http://127.0.0.1:8545", // Ganache default port