contracts-bedrock: be explicit with mintable erc721 token version

Previously any solidity version greater than 0.8.15 was allowed,
this updates it such that exactly 0.8.15 is allowed. This
matches the style of the other contracts in the system.

packages/contracts-bedrock/contracts/universal/OptimismMintableERC721.sol

 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.0;
+pragma solidity 0.8.15;
 
 import {
     ERC721Enumerable