RFR: 8365832: Refactor FloatingDecimal

[Shaojin Wen]

Since FloatToDecimal and DoubleToDecimal are used in Float.toString and Double.toString, some code in FloatingDecimal is not used. This refactoring is to remove the unused code and reuse DecimalDigits to reduce duplication of code.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into floating_dec_202501
 - remove unused code
 - char[] -> byte[]

Changes: https://git.openjdk.org/jdk/pull/23311/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23311&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365832
  Stats: 202 lines in 2 files changed: 10 ins; 130 del; 62 mod
  Patch: https://git.openjdk.org/jdk/pull/23311.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23311/head:pull/23311

PR: https://git.openjdk.org/jdk/pull/23311