Schema validation: Incorrect String Format Regexp for date-time

[Cr4psy]

It appears that the regular expression used for the date time is not exactly following the standard as defined here

Indeed, incorrect timestamp as shown below will still be valid with the current implementation:
2022-02-22T11:22:33:999Z
2022-02-22T11:22:33-999Z
2022-02-22T11:22:33x999Z

[Cr4psy]

Proposed a fix in #493