Fix Date to W3C string converter

[JensSpanier]

Description

Currently date conversion is not handled right if the timezone is different to UTC (Z).

Example: I live in Germany (currently UTC+02:00). So if my local time is 12:00, it's 10:00 in UTC. The correct representation of this time as W3C (ISO 8601) string is 2023-08-08T10:00:00Z or 2023-08-08T12:00:00+02:00.

But if I call dateToW3CString with a date created with new Date() it's converted to 2023-08-08T12:00:00Z. But thats 14:00 in my timezone. With toISOString it's correctly 2023-08-08T10:00:00.000Z.

So instead of using this complex and incorrect way to convert a Date to a W3C (ISO 8601) string, we should use the built-in function toISOString.

Check relevant checkboxes

• [x] I've run tests (through npm test) and they passed
• [x] I generated a working Apple Wallet Pass after the change
• [x] Provided examples keep working after the change
• [ ] This improvement is or might be a breaking change

[alexandercerutti]

Hey @JensSpanier, thank you for your PR! I'll take a deep look at it very soon, but at a first look, it looks already promising!

In the meanwhile I'd like to thank you for your contribution!

[alexandercerutti]

Released in v3.1.0 🎉 Thank you very much @JensSpanier !