git: openjdk/jdk: 8308388: Update description of SourceVersion.RELEASE_21

[Joe Darcy]

Changeset: d77a4103
Author:    Joe Darcy <darcy at openjdk.org>
Date:      2023-05-22 16:30:37 +0000
URL:       https://git.openjdk.org/jdk/commit/d77a41033a162c2c9bf02d6b8da91f2825320ad8

8308388: Update description of SourceVersion.RELEASE_21

Reviewed-by: iris

! src/java.compiler/share/classes/javax/lang/model/SourceVersion.java