Render note about unmaintained status as admonition

Author: chriskrycho

README.md

@@ -15,7 +15,11 @@
   </a>
 </p>
 
-> <br/>**Volta is unmaintained.** We recommend migrating to [`mise`](https://mise.jdx.dev/). See [issue #2080](https://github.com/volta-cli/volta/issues/2080).<br/><br/>
+> [!IMPORTANT]
+> **Volta is unmaintained.** We recommend migrating to [`mise`](https://mise.jdx.dev/). See [issue #2080](https://github.com/volta-cli/volta/issues/2080).
+
+---
+
 
 **Fast:** Install and run any JS tool quickly and seamlessly! Volta is built in Rust and ships as a snappy static binary.