Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/vendor/optimism/ICrossDomainMessenger.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (vendor/optimism/ICrossDomainMessenger.sol)
+pragma solidity >0.5.0 <0.9.0;
+
+/**
+ * @title ICrossDomainMessenger
+ */
+interface ICrossDomainMessenger {
+    /**********
+     * Events *
+     **********/
+
+    event SentMessage(address indexed target, address sender, bytes message, uint256 messageNonce, uint256 gasLimit);
+    event RelayedMessage(bytes32 indexed msgHash);
+    event FailedRelayedMessage(bytes32 indexed msgHash);
+
+    /*************
+     * Variables *
+     *************/
+
+    function xDomainMessageSender() external view returns (address);
+
+    /********************
+     * Public Functions *
+     ********************/
+
+    /**
+     * Sends a cross domain message to the target messenger.
+     * @param _target Target contract address.
+     * @param _message Message to send to the target.
+     * @param _gasLimit Gas limit for the provided message.
+     */
+    function sendMessage(address _target, bytes calldata _message, uint32 _gasLimit) external;
+}

backend/lib/openzeppelin-contracts/contracts/vendor/optimism/LICENSE

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+(The MIT License)
+
+Copyright 2020-2021 Optimism
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.