Update documentation for bref/extra-php-extensions versioning

Author: mnapoli

docs/upgrading/v3.mdx

@@ -31,6 +31,10 @@ Update all Bref packages in your `composer.json` file from `^2` to `^3`, for exa
     Only update the versions for the packages you actually have in your project.
 </Callout>
 
+<Callout>
+    If you use [`bref/extra-php-extensions`](https://github.com/brefphp/extra-php-extensions), note that it jumps from v1 to v3 (there is no v2). The v1 version was used with Bref v2, and v3 is aligned with Bref v3.
+</Callout>
+
 Then run:
 
 ```bash

website/src/pages/news/03-bref-3.0.mdx

@@ -47,6 +47,7 @@ What did we break? **Almost nothing**, the upgrade should be smooth. Here are th
 - The `vendor/bin/bref` CLI is removed (it was already 90% removed in v2).
 - The SOAP extension is now opt-in (it had very little usage).
 - If you deploy using container images, you'll need to update your Dockerfile.
+- If you use [`bref/extra-php-extensions`](https://github.com/brefphp/extra-php-extensions), update it from v1 to v3 (there is no v2 — v1 was for Bref v2, v3 is aligned with Bref v3).
 
 For a complete list of changes and instructions, check out the [**v3 upgrade guide**](../docs/upgrading/v3.mdx).