clarify whether to validate age of WNM pubtime

[tomkralidis]

WNM properties.pubtime is defined as:

          Identifies the date/time of when the file was posted/published, in RFC3339 format.
          The publication date/time is critical for subscribers to prevent message loss by knowing
          their lag (how far behind the publisher they are).

As discussed at the WIS2 Global Services meeting on 2024-05-21, we should consider whether to enforce a max-age requirement for properties.pubtime, in support of preventing publication of old/stale messages.

cc @hhaddouch @kaiwirt

[6a6d74]

See also wis2-notification-message/issue#120