fix: Versioning-safe implementation of placeholder inheritance

[fsbraun]

Description

The ContentRenderer.render_page_placeholders implementation is not versioning-safe, i.e. assumes that there is exactly one PageContent object per Page and language. This PR improves the implementation using the admin_manager.

This will allow {% placeholder "my_placeholder" inherit %} to also work with versioned page contents, such as provided by djangocms-versioning. (For it to actually work with djangocms-versioning, however, an additional change to djangocms-versioning is needed, to not deactivate the inherit functionality.)

Results with djangocms-versioning:

• When rendered at the URL (i.e. public endpoint): The placeholder only inherits from also public placeholders
• When previewed: The placeholder inherits from the latest parent placeholder (i.e. public or draft)
• When edited: Inherited placeholders are empty to allow for adding content

Then changes are covered by tests. Additional tests need to go into djangocms-versioning if the inheritance feature is activated again.

Related resources

• [BUG] placeholder inherit does not work djangocms-versioning#410
• #...

Checklist

• I have opened this pull request against develop-4
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined the channel #pr-reviews on our Discord Server to find a “pr review buddy” who is going to review my pull request.

[vinitkumar]

Looks good to me. Let's get this merged and released 🚢 🚀