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/utils/Context.sol

@@ -12,8 +12,6 @@ pragma solidity ^0.8.20;
  * is concerned).
  *
  * This contract is only required for intermediate, library-like contracts.
- *
- * @custom:stateless
  */
 abstract contract Context {
     function _msgSender() internal view virtual returns (address) {

contracts/utils/Multicall.sol

@@ -7,8 +7,6 @@ import {Address} from "./Address.sol";
 
 /**
  * @dev Provides a function to batch together multiple calls in a single external call.
- *
- * @custom:stateless
  */
 abstract contract Multicall {
     /**

contracts/utils/introspection/ERC165.sol

@@ -16,8 +16,6 @@ import {IERC165} from "./IERC165.sol";
  *     return interfaceId == type(MyInterface).interfaceId || super.supportsInterface(interfaceId);
  * }
  * ```
- *
- * @custom:stateless
  */
 abstract contract ERC165 is IERC165 {
     /**