fixes requested by limechain audit

contracts/DATAv2.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity >=0.8.0;
+pragma solidity 0.8.6;
 
 import "@openzeppelin/contracts/access/AccessControl.sol";
 import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Burnable.sol";

contracts/DataTokenMigrator.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity >=0.8.0;
+pragma solidity 0.8.6;
 
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
@@ -30,7 +30,7 @@ contract DataTokenMigrator { // is UpgradeAgent, see Crowdsale.sol
      * @param _from token holder that called CrowdsaleToken.upgrade(_value)
      * @param _value amount of tokens to upgrade, checked by the CrowdsaleToken
      */
-    function upgradeFrom(address _from, uint256 _value) public {
+    function upgradeFrom(address _from, uint256 _value) external {
         require(
             msg.sender == address(oldToken),
             "Call not permitted, UpgradableToken only"

contracts/IERC677.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity 0.8.6;
 
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 

contracts/IERC677Receiver.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity 0.8.6;
 
 interface IERC677Receiver {
     function onTokenTransfer(

contracts/MockRecipient.sol

@@ -1,5 +1,5 @@
 //SPDX-License-Identifier: Unlicense
-pragma solidity ^0.8.0;
+pragma solidity 0.8.6;
 
 import "hardhat/console.sol";
 import "./IERC677Receiver.sol";

hardhat.config.js

@@ -25,7 +25,7 @@ module.exports = {
     solidity: {
         compilers: [
             {
-                version: "0.8.3",
+                version: "0.8.6",
             },
             {
                 version: "0.4.11"