Update solidity version.

Author: MoMannn

README.md

@@ -78,7 +78,7 @@ You have created and have possession of unique glass-blown artwork (each having
 To do this, simply paste the code below into Remix and deploy the smart contract. You will "mint" a token for each new piece of artwork you want to see. Then you will "burn" that token when you surrender physical possession of the piece.
 
 ```solidity
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 import "https://github.com/0xcert/ethereum-erc721/src/contracts/tokens/nf-token-metadata.sol";
 import "https://github.com/0xcert/ethereum-erc721/src/contracts/ownership/ownable.sol";

hardhat.config.js

@@ -6,7 +6,7 @@ require('solidity-coverage');
  * @type import('hardhat/config').HardhatUserConfig
  */
 module.exports = {
-  solidity: '0.8.6',
+  solidity: '0.8.9',
   networks: {
     hardhat: {
       initialBaseFeePerGas: 0 // hardhat london fork error fix for coverage

src/contracts/mocks/nf-token-enumerable-mock.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 import "../../contracts/tokens/nf-token-enumerable.sol";
 import "../ownership/ownable.sol";

src/contracts/mocks/nf-token-metadata-enumerable-mock.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 import "../tokens/nf-token-metadata.sol";
 import "../tokens/nf-token-enumerable.sol";

src/contracts/mocks/nf-token-metadata-mock.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 import "../tokens/nf-token-metadata.sol";
 import "../ownership/ownable.sol";

src/contracts/mocks/nf-token-mock.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 import "../../contracts/tokens/nf-token.sol";
 import "../ownership/ownable.sol";

src/contracts/ownership/ownable.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 /**
  * @dev The contract has an owner address, and provides basic authorization control whitch

src/contracts/tokens/erc721-enumerable.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 /**
  * @dev Optional enumeration extension for ERC-721 non-fungible token standard.

src/contracts/tokens/erc721-metadata.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 /**
  * @dev Optional metadata extension for ERC-721 non-fungible token standard.

src/contracts/tokens/erc721-token-receiver.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.6;
+pragma solidity ^0.8.0;
 
 /**
  * @dev ERC-721 interface for accepting safe transfers.