RFR: 8337832: Optimize datetime toString

[Shaojin Wen]

Similar to PR #20321, this improves performance by providing a method that passes in a StringBuilder to avoid unnecessary object allocation.

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Commit messages:
 - copyright
 - use var
 - Update src/java.base/share/classes/java/time/ZonedDateTime.java
 - refactor datetime to string

Changes: https://git.openjdk.org/jdk/pull/20368/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20368&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337832
  Stats: 32 lines in 4 files changed: 23 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/20368.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20368/head:pull/20368

PR: https://git.openjdk.org/jdk/pull/20368