@Alisson Sousa Thanks for your patience on this.
I have checked with App Service Engineering team on this and sharing the below.
Java 21 on app service is expected to be available in next weeks and currently we don't have any ETA and it might changes based on different factors. You follow these Azure updates | Microsoft Azure for latest updates on App services.
For more information, you can refer to this blog post on Announcing Microsoft build on Openjdk21.
Feel free to reach back to me if you have any further questions on this.