Upgrade-safe storage for Initializable contract

[Ramarti]

Remove _gaps in favor of namespaced storage, following https://eips.ethereum.org/EIPS/eip-7201

contract AbcUpgradeable {
  // keccak256(keccak256("storyprotocol.storage.Abc") - 1) = 0x123...
  bytes32 private AbcStorageLocation = 0x123...;

	/// @custom:oz-unsafe-allow immutable
  uint immutable z;

  /// @custom:storage-location ercXXXX:openzeppelin.storage.Abc
  struct AbcStorage {
    uint x;
    uint y;
  }

	function _getAbcStorage() private pure returns (AbcStorage storage $) {
    assembly {
      $.slot := AbcStorageLocation
    }
  }

  function _foo() internal view {
    AbcStorage storage $ = _getAbcStorage();
    $.x.sendValue($.y);
  }
}