fix JDK version for native image

Author: PhilippSalvisberg

standalone/README.md

@@ -29,7 +29,7 @@ The parameters are the same as for the [SQLcl command `tvdformat`](../sqlcl/READ
 
 ### Native Image
 
-A native image is a platform specific executable. It does not require a JDK at runtime. A native image uses less resources and is faster. The following images can be produced with a GraalVM JDK 17:
+A native image is a platform specific executable. It does not require a JDK at runtime. A native image uses less resources and is faster. The following images can be produced with a GraalVM JDK 21:
 
 | OS      | amd64 (Intel)? | aarch64 (ARM)? |
 |---------|:--------------:|:--------------:|