javaversion: Update comments

[Bruno Haible]

This patch documents the possible values of the Java version, seen so far.
Tested with OpenJDK 9 .. 20.


2023-06-09  Bruno Haible  <bruno@clisp.org>

        javaversion: Update comments.
        * lib/javaversion.h (javaexec_version): Document the possible results up
        to OpenJDK 20.

diff --git a/lib/javaversion.h b/lib/javaversion.h
index 99157466b9..64cdbeaa32 100644
--- a/lib/javaversion.h
+++ b/lib/javaversion.h
@@ -33,7 +33,8 @@ extern "C" {
 
 /* Return information about the Java version used by execute_java_class().
    This is the value of System.getProperty("java.specification.version").
-   Some possible values are: 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 9, 10, 11.
+   Some possible values are: 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 9, 10, 11,
+   12, 13, 14, 15, 16, 17, 18, 19, 20.
    Return NULL if the Java version cannot be determined.  */
 extern char * javaexec_version (void)
   _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE;