chore: Re-license certain interface files (#890)

Licensing these as BUSL can create confusion and potential legal
ambiguity for third-parties who are simply trying to integrate existing
deployments that are operated under the Across protocol.

contracts/interfaces/HubPoolInterface.sol

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: BUSL-1.1
+// SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

contracts/interfaces/LpTokenFactoryInterface.sol

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: BUSL-1.1
+// SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 
 /**

contracts/interfaces/SpokePoolInterface.sol

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: BUSL-1.1
+// SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 
 /**

contracts/interfaces/SpokePoolMessageHandler.sol

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: BUSL-1.1
+// SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 
 // This interface is expected to be implemented by any contract that expects to receive messages from the SpokePool.

contracts/interfaces/V3SpokePoolInterface.sol

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: BUSL-1.1
+// SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 
 // Contains structs and functions used by SpokePool contracts to facilitate universal settlement.