`_setBaseURI` is deprecated in base contract

[yusefnapora]

The _setBaseURI function from the OpenZeppelin base contract has been removed and isn't present in versions > 3.4.0 (I think).

It looks like we should override the _baseURI method instead: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/3dc374ddcf46739185bd12759eaa826b579e5a0a/contracts/token/ERC721/ERC721.sol#L102

This will break if we update the base contract, so worth fixing :)