Incorrect __gap Size in ERC721Bridge.sol

**Bug Description**

The `__gap` size in `ERC721Bridge.sol` is incorrect. Given that the total consumed storage is 3 slots (2 bytes + 30 bytes, 20 bytes, and 20 bytes), the gap should be 47, not 46, to align with the rule that the next contract storage slot should start at a 50-slot boundary.

Breakdown of Storage Variables per Slot:
- 1st Slot: [uint8 _initialized](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/ecd2ca2cd7cac116f7a37d0e474bbb3d7d5e1c4d/contracts/proxy/utils/Initializable.sol#L62), [bool _initializing](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/ecd2ca2cd7cac116f7a37d0e474bbb3d7d5e1c4d/contracts/proxy/utils/Initializable.sol#L67), [bytes30 spacer_0_2_30](https://github.com/ethereum-optimism/optimism/blob/op-contracts-v180-blueprints-script/packages/contracts-bedrock/src/universal/ERC721Bridge.sol#L13) (Total: 32 bytes)
- 2nd Slot: [address messenger](https://github.com/ethereum-optimism/optimism/blob/op-contracts-v180-blueprints-script/packages/contracts-bedrock/src/universal/ERC721Bridge.sol#L17) (20 bytes)
- 3rd Slot: [address otherBridge](https://github.com/ethereum-optimism/optimism/blob/op-contracts-v180-blueprints-script/packages/contracts-bedrock/src/universal/ERC721Bridge.sol#L21) (20 bytes)

**Steps to Reproduce**

no steps

**Expected behavior**

Update the storage gap from `46` to `47` to ensure proper alignment with the storage slot allocation rules.

**Environment Information:**

Contract Version Reference
- OpenZeppelin: ecd2ca2cd7cac116f7a37d0e474bbb3d7d5e1c4d
- OP Contracts: v180-blueprints-script (latest?)

**Configurations:**

**Logs:**

**Additional context**

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⚠️ **Notice:** Issues that do not include the following sections will be subject to closure:
- Bug Description
- Steps to Reproduce
- Environment Information

Please ensure all required sections are filled out accurately to expedite the debugging process and improve issue resolution efficiency.

Start in packages/contracts-bedrock/src/universal/ERC721Bridge.sol and inspect the storage declarations and __gap value. Compare the documented three-slot layout with the intended 50-slot boundary. Done means the gap value and related storage-layout expectations are consistent.

Written by the indexing model from the issue text.