feat: make OptimismMintableUpgradableERC20.sol upgradable

packages/contracts-bedrock/contracts/universal/OptimismMintableUpgradableERC20.sol

@@ -1,29 +1,39 @@
 // SPDX-License-Identifier: MIT
 pragma solidity 0.8.15;
 
-import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import { ERC20Upgradeable } from "@openzeppelin/contracts-upgradeable/token/ERC20/ERC20Upgradeable.sol";
 import { IERC165 } from "@openzeppelin/contracts/utils/introspection/IERC165.sol";
+import { Initializable } from "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";
 import { ILegacyMintableERC20, IOptimismMintableERC20 } from "./IOptimismMintableERC20.sol";
-import { Semver } from "../universal/Semver.sol";
+
+contract SimpleSemver {
+    function version() public view returns (string memory) {
+        return string(abi.encodePacked("1.0.0"));
+    }
+}
 
 /**
- * @title OptimismMintableERC20
- * @notice OptimismMintableERC20 is a standard extension of the base ERC20 token contract designed
+ * @title OptimismMintableUpgradableERC20
+ * @notice OptimismMintableUpgradableERC20 is a standard extension of the base ERC20 token contract designed
  *         to allow the StandardBridge contracts to mint and burn tokens. This makes it possible to
  *         use an OptimismMintablERC20 as the L2 representation of an L1 token, or vice-versa.
  *         Designed to be backwards compatible with the older StandardL2ERC20 token which was only
  *         meant for use on L2.
  */
-contract OptimismMintableERC20 is IOptimismMintableERC20, ILegacyMintableERC20, ERC20, Semver {
+contract OptimismMintableUpgradableERC20 is IOptimismMintableERC20, ILegacyMintableERC20, Initializable, ERC20Upgradeable, SimpleSemver {
     /**
      * @notice Address of the corresponding version of this token on the remote chain.
+     * @notice Breaking the naming convention to remain as close as possible to the non-
+     *         upgradable version (where this is immutable)
      */
-    address public immutable REMOTE_TOKEN;
+    address public REMOTE_TOKEN;
 
     /**
      * @notice Address of the StandardBridge on this network.
+     * @notice Breaking the naming convention to remain as close as possible to the non-
+     *         upgradable version (where this is immutable)
      */
-    address public immutable BRIDGE;
+    address public BRIDGE;
 
     /**
      * @notice Emitted whenever tokens are minted for an account.
@@ -49,6 +59,10 @@ contract OptimismMintableERC20 is IOptimismMintableERC20, ILegacyMintableERC20,
         _;
     }
 
+    constructor() {
+        _disableInitializers();
+    }
+
     /**
      * @custom:semver 1.0.0
      *
@@ -57,12 +71,13 @@ contract OptimismMintableERC20 is IOptimismMintableERC20, ILegacyMintableERC20,
      * @param _name        ERC20 name.
      * @param _symbol      ERC20 symbol.
      */
-    constructor(
+    function initialize(
         address _bridge,
         address _remoteToken,
         string memory _name,
         string memory _symbol
-    ) ERC20(_name, _symbol) Semver(1, 0, 0) {
+    ) initializer public {
+        __ERC20_init(_name, _symbol);
         REMOTE_TOKEN = _remoteToken;
         BRIDGE = _bridge;
     }