Bugfix: Use Locale.ROOT for ISO8601 date formatting to prevent localized digits

[fhasse95]

This PR fixes a bug where Date.ISO8601Format() would produce localized ISO 8601 strings depending on the user's Android device locale instead of a stable, locale-independent representation.

The root of the issue was that the previous implementation used Java's default locale (java.util.Locale.getDefault()). On devices set to languages like Arabic or Hindi, however, SimpleDateFormat automatically localized the output:

• It converted numbers to non-ASCII digits (e.g., ٢٠٢٦ instead of 2026)
• It shifted years based on alternative calendars

Example:

iOS:

Android:

As you can see here, the digits of the date string were converted to Eastern Arabic numerals before the bugfix, whereas on iOS the ISO 8601 representation remained the same in every locale.

Since I use these date strings as timestamps in SkipSQL, this localization broke my database layer and caused record deserialization to fail as soon as the system language was changed.

To fix the issue, I replaced java.util.Locale.getDefault() with java.util.Locale.ROOT. Similar to Swift's en_US_POSIX, it avoids locale-specific number systems and ensures the output uses standard ASCII digits (0–9), making the ISO 8601 representation consistent across Android and iOS regardless of the user's device locale.

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[marcprux]

Great fix, thanks!