use solidity 0.8.18 for hardhat compatibility

contracts/StakeManager.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity 0.8.20;
+pragma solidity 0.8.18;
 
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 import "@openzeppelin/contracts/access/Ownable.sol";

contracts/StakeVault.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity 0.8.20;
+pragma solidity 0.8.18;
 
 import "./StakeManager.sol";
 import "@openzeppelin/contracts/access/Ownable.sol";

contracts/Token.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity 0.8.20;
+pragma solidity 0.8.18;
 
 import "@openzeppelin/contracts/access/Ownable.sol";
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contracts/legacy/Owned.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 /// @dev `Owned` is a base level contract that assigns an `owner` that can be
 ///  later changed
 contract Owned {

contracts/legacy/SNTFaucet.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 
 import "./SNTPlaceHolder.sol";
 

contracts/legacy/SNTPlaceHolder.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 import "./token/TokenController.sol";
 import "./token/MiniMeToken.sol";
 import "./SafeMath.sol";
@@ -31,7 +31,7 @@ import "./Owned.sol";
 ///  asks it to do so.
 contract SNTPlaceHolder is TokenController, Owned {
     using SafeMath for uint256;
-    MiniMeToken public immutable snt;
+    MiniMeToken public snt;
 
     constructor(address _owner, address payable _snt) {
         owner = _owner;

contracts/legacy/SafeMath.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 
 /**
  * Math operations with safety checks

contracts/legacy/token/ApproveAndCallFallBack.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 abstract contract ApproveAndCallFallBack {
     function receiveApproval(address from, uint256 _amount, address _token, bytes memory _data) virtual public;
 }

contracts/legacy/token/Controlled.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 
 contract Controlled {
     string internal constant ERR_BAD_PARAMETER = "Bad parameter";

contracts/legacy/token/MiniMeToken.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 /*
     Copyright 2016, Jordi Baylina
 
@@ -39,9 +39,9 @@ import "./MiniMeTokenFactory.sol";
 contract MiniMeToken is Controlled {
 
     string public name;                //The Token's name: e.g. DigixDAO Tokens
-    uint8 public immutable decimals;             //Number of decimals of the smallest unit
+    uint8 public decimals;             //Number of decimals of the smallest unit
     string public symbol;              //An identifier: e.g. REP
-    string public constant version = "MMT_0.1"; //An arbitrary versioning scheme
+    string public version = "MMT_0.1"; //An arbitrary versioning scheme
 
     /**
      * @dev `Checkpoint` is the structure that attaches a block number to a
@@ -59,14 +59,14 @@ contract MiniMeToken is Controlled {
 
     // `parentToken` is the Token address that was cloned to produce this token;
     //  it will be 0x0 for a token that was not cloned
-    MiniMeToken public immutable parentToken;
+    MiniMeToken public parentToken;
 
     // `parentSnapShotBlock` is the block number from the Parent Token that was
     //  used to determine the initial distribution of the Clone Token
-    uint public immutable parentSnapShotBlock;
+    uint public parentSnapShotBlock;
 
     // `creationBlock` is the block number that the Clone Token was created
-    uint public immutable creationBlock;
+    uint public creationBlock;
 
     // `balances` is the map that tracks the balance of each address, in this
     //  contract when the balance changes the block number that the change
@@ -83,7 +83,7 @@ contract MiniMeToken is Controlled {
     bool public transfersEnabled;
 
     // The factory used to create new clone tokens
-    MiniMeTokenFactory public immutable tokenFactory;
+    MiniMeTokenFactory public tokenFactory;
 
 ////////////////
 // Constructor

contracts/legacy/token/MiniMeTokenFactory.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 import "./MiniMeToken.sol";
 
 ////////////////

contracts/legacy/token/TokenController.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity 0.4.20;
+pragma solidity 0.8.18;
 /**
  * @dev The token controller contract must implement these functions
  */

hardhat.config.js

@@ -2,7 +2,7 @@ require("@nomicfoundation/hardhat-toolbox");
 
 /** @type import('hardhat/config').HardhatUserConfig */
 module.exports = {
-  solidity: "0.8.20",
+  solidity: "0.8.18",
   mocha: {
     timeout: 100000000,
   }