Archive Classic IQP migration guide (#4145)

This PR adds a small note to inform the users that the guide will be
archived.

Author: arnaucasau

docs/migration-guides/classic-iqp-to-cloud-iqp.mdx

@@ -3,6 +3,10 @@ title: Migrate to the upgraded IBM Quantum Platform
 description: Migrate to using the upgraded IBM Quantum Platform for Open users
 ---
 
+<Admonition type="note">
+This migration guide is no longer maintained. It is archived in GitHub for historical reference only.
+</Admonition>
+
 # Migrate to the upgraded IBM Quantum Platform
 
 This guide describes how, why, and when to transition to the [new version of IBM Quantum Platform.](https://quantum.cloud.ibm.com/)