RFR: 8210336: DateTimeFormatter predefined formatters should support short time zone offsets

[Naoto Sato]

This PR is a follow-on fix to [JDK-8032051](https://bugs.openjdk.org/browse/JDK-8032051), where it allowed short offsets only for ZonedDateTime parsing. This fix allows all predefined ISO formatters to accept short offsets. A corresponding CSR has been drafted.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/29393/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8210336
  Stats: 53 lines in 3 files changed: 49 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29393.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29393/head:pull/29393

PR: https://git.openjdk.org/jdk/pull/29393