Swap from React Helmet to Unhead (#38896)

app/javascript/mastodon/features/about/index.jsx

@@ -3,7 +3,7 @@ import { PureComponent } from 'react';
 
 import { defineMessages, FormattedMessage } from 'react-intl';
 
-import { Helmet } from 'react-helmet';
+import { Helmet } from '@unhead/react/helmet';
 
 import ImmutablePropTypes from 'react-immutable-proptypes';
 import { connect } from 'react-redux';