RFR: 8348880: Replace ConcurrentMap with AtomicReferenceArray for ZoneOffset.MINUTES_15_CACHE

[Shaojin Wen]

ZoneOffset.MINUTES_15_CACHE uses AtomicReferenceArray to replace ConcurrentMap to avoid object allocation caused by boxing from int to Integer during access.

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Commit messages:
 - Update src/java.base/share/classes/java/time/ZoneOffset.java
 - MINUTES_15_CACHE use AtomicReferenceArray instead of ConcurrentMap

Changes: https://git.openjdk.org/jdk/pull/23337/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23337&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348880
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/23337.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23337/head:pull/23337

PR: https://git.openjdk.org/jdk/pull/23337