RFR: 8336792: DateTimeFormatterBuilder append zeros based on StringBuilder.repeat

[Shaojin Wen]

The StringBuilder.repeat method was added in JDK 21, which can be used to make some minor optimizations to existing code, including DateTimeFormatterBuilder

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Commit messages:
 - use StringBuilder#repeat append zeros

Changes: https://git.openjdk.org/jdk/pull/20245/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20245&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336792
  Stats: 12 lines in 1 file changed: 2 ins; 4 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20245.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20245/head:pull/20245

PR: https://git.openjdk.org/jdk/pull/20245