Summary
-------
Update core libraries for JDK 21 by adding an enum constant to `ClassFileFormatVersion`.

Problem
-------

With a new release, `ClassFileFormatVersion` needs a constant to model that release.

Solution
--------

Add the new constant.

Specification
-------------

    --- a/src/java.base/share/classes/java/lang/reflect/ClassFileFormatVersion.java
    +++ b/src/java.base/share/classes/java/lang/reflect/ClassFileFormatVersion.java
    @@ -262,7 +262,17 @@ public enum ClassFileFormatVersion {
          * href="https://docs.oracle.com/javase/specs/jvms/se20/html/index.html">
          * <cite>The Java Virtual Machine Specification, Java SE 20 Edition</cite></a>
          */
    -    RELEASE_20(64);
    +    RELEASE_20(64),
    +
    +    /**
    +     * The version recognized by the Java Platform, Standard Edition
    +     * 21.
    +     *
    +     * @see <a
    +     * href="https://docs.oracle.com/javase/specs/jvms/se21/html/index.html">
    +     * <cite>The Java Virtual Machine Specification, Java SE 21 Edition</cite></a>
    +     */
    +    RELEASE_21(65);
     
         // Note to maintainers: when adding constants for newer releases,
         // the implementation of latest() must be updated too.
    @@ -277,7 +287,7 @@ public enum ClassFileFormatVersion {
          * {@return the latest class file format version}
          */
         public static ClassFileFormatVersion latest() {
    -        return RELEASE_20;
    +        return RELEASE_21;
         }
     
         /**