Swap from React Helmet to Unhead (#38896)

app/javascript/mastodon/features/ui/components/modal_root.jsx

@@ -1,7 +1,7 @@
 import PropTypes from 'prop-types';
 import { PureComponent } from 'react';
 
-import { Helmet } from 'react-helmet';
+import { Helmet } from '@unhead/react/helmet';
 
 import Base from 'mastodon/components/modal_root';
 import { AltTextModal } from 'mastodon/features/alt_text_modal';