Update remappings.txt for upgradeable contracts and set up submodule (#4639)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Ernesto García <ernestognw@gmail.com>

contracts/token/ERC1155/utils/ERC1155Holder.sol

@@ -11,8 +11,6 @@ import {IERC1155Receiver} from "../IERC1155Receiver.sol";
  *
  * IMPORTANT: When inheriting this contract, you must include a way to use the received tokens, otherwise they will be
  * stuck.
- *
- * @custom:stateless
  */
 abstract contract ERC1155Holder is ERC165, IERC1155Receiver {
     /**

contracts/token/ERC721/utils/ERC721Holder.sol

@@ -11,8 +11,6 @@ import {IERC721Receiver} from "../IERC721Receiver.sol";
  * Accepts all token transfers.
  * Make sure the contract is able to use its token with {IERC721-safeTransferFrom}, {IERC721-approve} or
  * {IERC721-setApprovalForAll}.
- *
- * @custom:stateless
  */
 abstract contract ERC721Holder is IERC721Receiver {
     /**