Rename JSX files with proper .jsx extension (#23733)

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

@@ -0,0 +1,11 @@
+import React from 'react';
+import Column from '../ui/components/column';
+import MissingIndicator from '../../components/missing_indicator';
+
+const GenericNotFound = () => (
+  <Column>
+    <MissingIndicator fullPage />
+  </Column>
+);
+
+export default GenericNotFound;