RFR: 8339803: Acknowledge case insensitive unambiguous keywords in tzdata files

[Naoto Sato]

This is a follow on fix to [JDK-8339644](https://bugs.openjdk.org/browse/JDK-8339644). It turned out that TZ data files allow abbreviation of keywords, such as "ZO" for "Zone." Same fix, i.e, replacing `startsWith()` with `regionMatches()` was applied.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/20940/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20940&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339803
  Stats: 41 lines in 4 files changed: 7 ins; 4 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/20940.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20940/head:pull/20940

PR: https://git.openjdk.org/jdk/pull/20940