Simpler copy for README closed beta callout

Author: RomainCscn

README.md

@@ -18,8 +18,8 @@
   <a href="https://github.com/linear/linear-release/blob/main/LICENSE"><img src="https://img.shields.io/badge/license-MIT-blue.svg" alt="Linear Release is released under the MIT license."></a>
 </p>
 
-> [!NOTE]
-> This project is currently in beta and requires enrollment to use. If you're interested in trying it out or need assistance, please contact [Linear support](https://linear.app/contact) or your account manager. APIs and commands may change in future releases.
+> [!IMPORTANT]
+> Linear Release is in closed beta. Contact [Linear support](https://linear.app/contact) or your account manager to request access. APIs and commands may change.
 
 ## Overview