Make the legacy XmlConvert DateTime format explicit

Replace resolved XmlConvert.ToString(DateTime) with its exact historical string-format overload, preserving the seven fractional digits and offset formatting rather than selecting a serialization mode. Retain the original DateTime expression and evaluation. Exclude other overloads/types, dynamic/custom conversions, named arguments, method groups, lambda/query bodies, files with generic declarations and conditional snapshots. Opt-in compatibility formatting; not in version composites.