Summary
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DecimalFormat uses a double->string algorithm that was used in releases prior to JDK 21. Meanwhile, Formatter and Double.toString() use a newer algorithm introduced in JDK 21.
In very rare cases, this leads to slightly different outcomes between DecimalFormat and Formatter with equivalent format specifications when formatting doubles. 

Problem
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In rare cases, DecimalFormat and Formatter produce slightly different outcomes on some doubles.
For example, currently the `double` 7.3879E20 is output as "738790000000000100000" with DecimalFormat and as "738790000000000000000" with Formatter, given equivalent format specifications.
With this change, the outcomes are the same.

Solution
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By using the same algorithm used by Double.toString() and Formatter, the behavior of DecimalFormat becomes aligned with these and produce comparable outcomes.

Specification
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There are no visible API changes. This is a behavioral change on those rare doubles that the old double->string algorithm converts slightly differently than the new algorithm.

To help migrating applications that might be impacted by this change, for a few releases the old algorithm will still be available to DecimalFormat and classes that depend on it.

The old algorithm can be enabled with the "-Djdk.compat.DecimalFormat=true" option on the launcher command line. Absent this option, the new algorithm is used.