RFR: 8290918: Initial nroff manpage generation for JDK 21

[David Holmes]

Please review this simple update to the manpage to set the version to 21-ea.

This change also corrects a typo in javac.1 that was manually introduced by JDK-8300591

Thanks.

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Commit messages:
 - 8290918: Initial nroff manpage generation for JDK 21

Changes: https://git.openjdk.org/jdk/pull/12154/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290918
  Stats: 28 lines in 27 files changed: 0 ins; 0 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/12154.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12154/head:pull/12154

PR: https://git.openjdk.org/jdk/pull/12154