Use ISO 8601 date format for formatting dates

[jonatan-ivanov]

ISO 8601 is the commonly accepted international date format. Using it makes it easy for humans to read dates and also machines to parse it.

Right now https://developers.events/all-cfps.json does not use it, it uses something else which also has some encoding issues in the json see:

"until": "10-Avr-2022",
"until": "21-Jan-2022-\u003E03-Mar-2022",
"until": "24-Jan-2022",

[jespino]

@scraly, this would probably be a good option for a new validator.