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

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

@@ -0,0 +1,16 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+
+const ColumnSubheading = ({ text }) => {
+  return (
+    <div className='column-subheading'>
+      {text}
+    </div>
+  );
+};
+
+ColumnSubheading.propTypes = {
+  text: PropTypes.string.isRequired,
+};
+
+export default ColumnSubheading;