Issue Summary

If you are trying to exclude related fields from being copied, the revision content ends up with a value of DeferringRelatedManager and throws an error on revision.save():

Object of type DeferringRelatedManager is not JSON serializable

Steps to Reproduce

1. Set up a model, say, blog and a related model author, so you have blog.authors.all()
2. Set BlogPage.exclude_fields_in_copy = ["authors"].
3. Try and copy a blog instance from Wagtail admin
4. 💥

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

• Python version: 3.11
• Django version: 5.0.2
• Wagtail version: 6.0.1

Working on this

I think the approach would be to properly exclude the field from the revision. Not sure why it's getting the value from page_copy -

https://github.com/wagtail/wagtail/blob/main/wagtail/actions/copy_page.py#L224-L230

This code here causes it to be set to a related manager value.

                    if exclude_field in revision_content and hasattr(
                        page_copy, exclude_field
                    ):
                        revision_content[exclude_field] = getattr(
                            page_copy, exclude_field, None
                        )