RFR: 8335645: j.u.Formatter#trailingZeros improved with String repeat

[Shaojin Wen]

In JDK 21, StringBuilder added a repeat method, which can be used to improve j.u.Formatter#trailingZeros

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Commit messages:
 - use StringBuilder#repeat

Changes: https://git.openjdk.org/jdk/pull/20018/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335645
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20018.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20018/head:pull/20018

PR: https://git.openjdk.org/jdk/pull/20018