RFR: 8344646: The libjsig deprecation warning should go to stderr not stdout

[David Holmes]

In JDK 16 we deprecated the old signal and sigset signal-chaining interfaces under [JDK-8257572](https://bugs.openjdk.org/browse/JDK-8257572). The deprecation warning was written to stdout but all other warnings go to stderr. Having the warning be on stdout causes interference with application output so we change it to stderr.

Testing
 - examined signal test output (see JBS issue)
 - tiers 1-3 sanity

Thanks

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Commit messages:
 - 8344646: The libjsig deprecation warning should go to stderr not stdout

Changes: https://git.openjdk.org/jdk/pull/22283/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22283&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344646
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/22283.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22283/head:pull/22283

PR: https://git.openjdk.org/jdk/pull/22283