RFR: JDK-8308388: Update description of SourceVersion.RELEASE_21

[Joe Darcy]

Update description of SourceVersion.RELEASE_21 for the new language features (once they are pushed).

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Commit messages:
 - JDK-8308388: Update description of SourceVersion.RELEASE_21

Changes: https://git.openjdk.org/jdk/pull/14051/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14051&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308388
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14051.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14051/head:pull/14051

PR: https://git.openjdk.org/jdk/pull/14051