docs: use gfm banner

Author: idleberg

README.md

@@ -27,7 +27,8 @@ gulp.task('lint', done => {
 });
 ```
 
-:warning: This plugin is now pure ESM. [Read how to migrate your `Gulpfile`](https://gist.github.com/noraj/007a943dc781dc8dd3198a29205bae04).
+> [!IMPORTANT]
+> This plugin is now pure ESM. [Read how to migrate your `Gulpfile`](https://gist.github.com/noraj/007a943dc781dc8dd3198a29205bae04). Alternatively, you can use any 1.x version.
 
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