Merge pull request #2571 from id/20250514-emqx-deprecation-notice

emqx: add deprecation notice

emqx/deprecated.md

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+Starting from v5.9.0, EMQX has unified all features from the previous Open Source and Enterprise editions into a single, powerful offering with the Business Source License (BSL) 1.1.
+
+If you want to understand why we made the change, please read this [blog post](https://www.emqx.com/en/news/emqx-adopts-business-source-license), and if you want to know more about the new license, please read the [EMQX Licensing FAQ](https://www.emqx.com/en/content/license-faq).
+
+Consequently, we stopped publishing the `emqx` Docker Official Image. EMQX v5.9.0+ will only be available in the [`emqx/emqx`](https://hub.docker.com/r/emqx/emqx) and [`emqx/emqx-enterprise`](https://hub.docker.com/r/emqx/emqx-enterprise) Docker Hub repositories.