Don't overwrite ancestor configurations for public access

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #17709

Description

See the linked issue for a description and steps to reproduce 😃

Testing this PR

Verify that:

1. Public access can be added and updated in the structure below a protected document, without affecting the ancestor protection.
2. Public access can be removed from the structure below a protected document, without affecting the ancestor protection.
3. Public access can be removed from an ancestor document, without affecting any descendants with explicit public access configuration.