title: Using Strings for uint256 not working with ERC721EnumerableUpgradeable
body: <p>just like the title says I cannot get using Strings to compile, I keep getting the identifier not found.
  This is the start of my .sol contract</p>
  <pre><code>import &quot;@openzeppelin/contracts-upgradeable/token/ERC721/extensions/ERC721EnumerableUpgradeable.sol&quot;;
  import &quot;@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol&quot;;
  import &quot;@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol&quot;;
  import &quot;@openzeppelin/contracts-upgradeable/utils/StringsUpgradeable.sol&quot;;
  
  contract NFT is Initializable, StringsUpgradeable, ERC721EnumerableUpgradeable, OwnableUpgradeable {
  using Strings for uint256; 
  </code></pre>
  <p>Please help!</p>
  <p>Thank you!</p>