Possible to get around max_count by converting aliases

Issue Summary

I believe that it is possible to get around the max_count and max_count_per_parent restrictions on pages by copying pages as aliases, and then converting those aliases to ordinary pages. https://docs.wagtail.org/en/v7.0.2/reference/models.html#wagtail.models.Page.max_count

Steps to Reproduce

1. Start a new project with wagtail start myproject
2. Edit home/models.py:

class HomePage(Page):
    pass

class ChildPage(Page):
    max_count = 1

1. In the admin interface create a ChildPage instance under the HomePage
2. In the admin interface, create an copy to create an alias of the ChildPage. (Note that a non-alias copy fails with a permission error)
3. Convert the alias into an ordinary page

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

Technical details

• Python version: 3.12.3
• Django version: 5.2.4
• Wagtail version: 7.0.2

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