RFR: JDK-8308388: Update description of SourceVersion.RELEASE_21 [v3]

[Iris Clark]

On Fri, 19 May 2023 21:23:54 GMT, Joe Darcy <darcy at openjdk.org> wrote:

>> Update description of SourceVersion.RELEASE_21 for the new language features (once they are pushed).
>
> Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update comment.

Marked as reviewed by iris (Reviewer).

-------------

PR Review: https://git.openjdk.org/jdk/pull/14051#pullrequestreview-1435432956