Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/proxy/beacon/BeaconProxy.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.7.0) (proxy/beacon/BeaconProxy.sol)
+
+pragma solidity ^0.8.0;
+
+import "./IBeacon.sol";
+import "../Proxy.sol";
+import "../ERC1967/ERC1967Upgrade.sol";
+
+/**
+ * @dev This contract implements a proxy that gets the implementation address for each call from an {UpgradeableBeacon}.
+ *
+ * The beacon address is stored in storage slot `uint256(keccak256('eip1967.proxy.beacon')) - 1`, so that it doesn't
+ * conflict with the storage layout of the implementation behind the proxy.
+ *
+ * _Available since v3.4._
+ */
+contract BeaconProxy is Proxy, ERC1967Upgrade {
+    /**
+     * @dev Initializes the proxy with `beacon`.
+     *
+     * If `data` is nonempty, it's used as data in a delegate call to the implementation returned by the beacon. This
+     * will typically be an encoded function call, and allows initializing the storage of the proxy like a Solidity
+     * constructor.
+     *
+     * Requirements:
+     *
+     * - `beacon` must be a contract with the interface {IBeacon}.
+     */
+    constructor(address beacon, bytes memory data) payable {
+        _upgradeBeaconToAndCall(beacon, data, false);
+    }
+
+    /**
+     * @dev Returns the current beacon address.
+     */
+    function _beacon() internal view virtual returns (address) {
+        return _getBeacon();
+    }
+
+    /**
+     * @dev Returns the current implementation address of the associated beacon.
+     */
+    function _implementation() internal view virtual override returns (address) {
+        return IBeacon(_getBeacon()).implementation();
+    }
+
+    /**
+     * @dev Changes the proxy to use a new beacon. Deprecated: see {_upgradeBeaconToAndCall}.
+     *
+     * If `data` is nonempty, it's used as data in a delegate call to the implementation returned by the beacon.
+     *
+     * Requirements:
+     *
+     * - `beacon` must be a contract.
+     * - The implementation returned by `beacon` must be a contract.
+     */
+    function _setBeacon(address beacon, bytes memory data) internal virtual {
+        _upgradeBeaconToAndCall(beacon, data, false);
+    }
+}