Since FloatToDecimal and DoubleToDecimal are used in Float.toString and Double.toString, some code in FloatingDecimal is not used. 
This PR refactors `FloatingDecimal` and `DigitList` to improve efficiency and reduce code duplication.
Key changes:
* Convert internal digit storage from `char[]` to `byte[]` for reduced memory footprint.
* Remove unused code and methods that are no longer needed.
            
This PR refactors `FloatingDecimal` and `DigitList` to improve efficiency and reduce code duplication.
Key changes:
* Convert internal digit storage from `char[]` to `byte[]` for reduced memory footprint.
* Remove unused code and methods that are no longer needed.
- links to
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                     Commit(master)
        openjdk/jdk/dba0d545 Commit(master)
        openjdk/jdk/dba0d545
- 
                     Review(master)
        openjdk/jdk/23311 Review(master)
        openjdk/jdk/23311
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