New API functions for date conversions

As requested in PR review, it is no longer possible to provide timestamp
(version date order) as an integer to REST API /projects/{{id}}/versions
endpoints.

It is worth noting that a Unix timestamp can technically still be given,
using the @{integer} syntax, since that is a valid format for the
DateTimeImmutable constructor. This could change in the future, if we
decide to only allow ISO-8601 format.

The datetime parsing and generation logic has been standardized in 2 new
date API functions:

• date_string_to_timestamp()
• date_timestamp_to_iso8601()

Use the new functions in:

• date_strtotime()
• ApiObjectFactory: methods datetime() and datetimeString();
  add new dateStringToTimestamp() method.
• VersionAddCommand
• VersionUpdateCommand

Fixes 0034928