Fix Rfc3339DateJsonAdapter to use UTC for date-only strings

[amitmishra11]

Fixes #2046

Bug

Rfc3339DateJsonAdapter uses the host machine's default time zone when
parsing a date string that has no time or timezone component (e.g.
"2025-11-20"). The parsing code in Iso8601Utils.kt contained a
comment that directly acknowledged this:

// Note that this uses the host machine's time zone. That's a bug.
return GregorianCalendar(year, month - 1, day).time

This means two machines in different time zones would produce different
Date values from the same JSON input, which is surprising and
inconsistent with how dates that carry an explicit Z suffix are
handled.

Fix

Replace the default-timezone GregorianCalendar constructor call with
an explicit UTC calendar, matching the pattern already used for the
full datetime path. The fix mirrors what the existing code does when a
timezone is present.

Before:

return GregorianCalendar(year, month - 1, day).time

After:

val calendar: Calendar = GregorianCalendar(TIMEZONE_Z, Locale.US)
calendar.isLenient = false
calendar[Calendar.YEAR] = year
calendar[Calendar.MONTH] = month - 1
calendar[Calendar.DAY_OF_MONTH] = day
calendar[Calendar.HOUR_OF_DAY] = 0
calendar[Calendar.MINUTE] = 0
calendar[Calendar.SECOND] = 0
calendar[Calendar.MILLISECOND] = 0
return calendar.time

Test changes

• The absentTimeZone test now asserts that "1970-01-01" produces
  the same Date as "1970-01-01Z" (midnight UTC).
• A second assertion covers the example from the issue: "2025-11-20".
• The newDateWithHostZone helper, which existed only to document the
  old (incorrect) behaviour, is removed.

Verification

./gradlew :moshi-adapters:test
BUILD SUCCESSFUL