Remove empty block from getting-started.md

Removed an unnecessary empty block in the "CAUTION" section of the documentation. This change cleans up the formatting and ensures the documentation is more readable and concise.

core/getting-started.md

@@ -5,7 +5,6 @@
 You can choose your preferred stack between Symfony, Laravel, or bootstrapping the API Platform core library manually.
 
 > [!CAUTION]
->
 > If you are migrating from an older version of API Platform, make sure you read the [Upgrade Guide](upgrade-guide.md).
 
 ### Symfony