* In [28481], `$admin_header_callback` and `$admin_image_div_callback` were set to `private` based on their erroneous `@param` values
* `$admin_header_callback` and `$admin_image_div_callback` are used as hook callbacks - as such, they must be `public`
* In [28521] and [28524], magic methods were added for back-compat
* Currently, there are 4 properties marked `private`: `$uploaded_headers`, `$default_headers`, `$page`, and `$updated` - `$page` and `$uploaded_headers` are never used and `$updated` was added by me in [30187] during 4.1. `$default_headers` does not necessarily need to be `private`
Set `$admin_header_callback` and `$admin_image_div_callback` to `public`.
Remove the `$page` property - it duplicated the `$page` local var and is referenced/used nowhere.
Remove the `$uploaded_headers` property - it is used nowhere and is dead code.
Set `$default_headers` to `public`.
Remove the magic methods - they were beyond overkill and rendered moot by the above changes.
See #30891.
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