RFR: JDK-8300698: Missing @since tag for ClassFileFormatVersion.RELEASE_21

[Joe Darcy]

The addition of the new enum constant for ClassFileFormatVersion.RELEASE_21 neglected to include an @since tag; this should be corrected.

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Commit messages:
 - JDK-8300698: Missing @since tag for ClassFileFormatVersion.RELEASE_21

Changes: https://git.openjdk.org/jdk/pull/12107/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12107&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300698
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12107.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12107/head:pull/12107

PR: https://git.openjdk.org/jdk/pull/12107