Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/governance/extensions/GovernorProposalThreshold.sol

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (governance/extensions/GovernorProposalThreshold.sol)
+
+pragma solidity ^0.8.0;
+
+import "../Governor.sol";
+
+/**
+ * @dev Extension of {Governor} for proposal restriction to token holders with a minimum balance.
+ *
+ * _Available since v4.3._
+ * _Deprecated since v4.4._
+ */
+abstract contract GovernorProposalThreshold is Governor {
+    function propose(
+        address[] memory targets,
+        uint256[] memory values,
+        bytes[] memory calldatas,
+        string memory description
+    ) public virtual override returns (uint256) {
+        return super.propose(targets, values, calldatas, description);
+    }
+}