TimeZoneInfo: Reduce intermediate allocations in GetTimeZoneIds on Unix (#8769)

TimeZoneInfo: Reduce intermediate allocations in GetTimeZoneIds on Unix (#8769)

Avoid intermediate underlying array allocations as items are added to
the resulting List<string> by specifying the capacity. Also, change
the signature of the private method to return List<string> instead of
IEnumerable<string> to avoid the enumerator allocation when enumerating
the ids.