date-time differences w.r.t. spec and JSON schema

[danielpeintner]

In our document we use date-time for example for created and modified
see https://w3c.github.io/wot-thing-description/#thing

In the TD spec we point to https://www.w3.org/TR/2012/REC-xmlschema11-2-20120405/#dateTime which in the end is consistent with ISO 8601.

On the other hand in our JSON schema we use RFC3339 date-time

wot-thing-description/validation/td-json-schema-validation.json

Lines 1398 to 1405 in 73e21a8

	"created": {
	"type": "string",
	"format": "date-time"
	},
	"modified": {
	"type": "string",
	"format": "date-time"
	},

There are examples that work for both, e.g, 2020-12-09T16:09:53+00:00 but there are variants that are not valid for both (e.g., without T between date and time).
see https://stackoverflow.com/a/65221179