Allow ISO 8601 timestamps with local time in `isoTimestamp` action

[marianheinsen]

The isoTimestamp action only allows timestamps with a timezone designator such as UTC or a time offset. However, local timestamps without timezone should also be considered valid.

https://en.wikipedia.org/wiki/ISO_8601#Local_time_(unqualified)

Example: 1995-03-31T00:00:00

I'm working with an external data source that is scoped to a specific country with a uniform timezone and does only contain timestamps without timezone information.

[muningis]

isoDateTime should be used instead - https://valibot.dev/api/isoDateTime/, however I see it does not accept seconds, only hours and minutes, which seems incompatible with JSON Schema 5.x - https://json-schema.org/understanding-json-schema/reference/type#dates-and-times