The name `ConvertDateTimeToUtc` confuses date formats and time zones

[BryanWilhite]

https://github.com/BryanWilhite/SonghayCore/blob/c9923e7371f17334707699ea6f292c5ed6311862/SonghayCore/ProgramTypeUtility._.cs#L82

UTC refers to a time zone; the ToString string template in ConvertDateTimeToUtc comes from the ISO 8601 format

• [x] add an extension method, DateTimeExtensions.ToIso8601String, that uses the string template ConvertDateTimeToUtc to convert to the ISO 8601 format 🔨 ✨
• [ ] mark ConvertDateTimeToUtc obsolete and refer to DateTimeExtensions.ToIso8601String 🔨 🧊
• [x] remove ToPrettyDate for extra credit 🍎 because it has been marked obsolete for a large time 🔨 🔥

🔗 https://en.wikipedia.org/wiki/ISO_8601

[BryanWilhite]

ConvertDateTimeToUtc is just a non-helpful wrapper around ToUniversalTime which should not be 'hidden' from readers of the code