RFR: 8371431: Warning message when turning off CompactStrings

[Shaojin Wen]

A warning message should be given before removing the CompactStrings off option.

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Commit messages:
 - Merge branch 'master' into compact_str_warn_2510
 - from @liach
 - add warnings

Changes: https://git.openjdk.org/jdk/pull/27995/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27995&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371431
  Stats: 13 lines in 3 files changed: 1 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27995.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27995/head:pull/27995

PR: https://git.openjdk.org/jdk/pull/27995