*: website_blog, website_event, website_event_track,
website_event_exhibitor, website_event_sale, website_hr_recruitment,
website_sale, website_slides

Since [1], when scheduling a "blog post" to be published on a
specific date (in the future) by changing the "publishing date", a
notification was sent to all blog followers directly, even though the
post was not yet accessible. Additionally, the sitemap.xml indicated
this post as available, which made crawlers index it even though it
returned a 404.

This commit introduces a new way of managing web publication scheduling
(blog posts, e-commerce products, web pages, events...) by modifying the
WebsitePublishedMixin and adding three fields:

The "published_date" field is added to default website pages, accurately
tracking the date and time of publication.

The "publish_on" field is introduced to track scheduled publication
dates. This field allows users to specify the date and time for
automatic publication of content, improving efficiency in disseminating
information while optimizing resource allocation.

It is now possible to use this functionality for all models inheriting
from this mixin. A "_check_for_action_post_publish" method has been
implemented to be replaced in child models to trigger a procedure just
before publishing a page and to record the "published_date". This
allows, for example, sending a notification to users only if the page
has never been published before.

This behavior is managed through the use of a "cron_trigger" linked to a
static cron that checks if a web page has a "publish_on" field in the
past. In this case, the page is published and calls the function to
publish the page.

Upgrade PR :
- https://github.com/odoo/upgrade/pull/5903

[1]: https://github.com/odoo/odoo/commit/a4d8cf9

task-3650652