Replace deprecated `who` argument from user query

[grappler]

What problem does this address?

In WordPress 5.9 the who argument has been deprecated. Trac Ticket

Gutenberg is still using the who argument and recommending it to be used. https://github.com/WordPress/gutenberg/blob/08cf4a96d1f14fcdf4c55886ba237fb294f9c465/packages/editor/src/components/post-author/constants.js#L2 https://github.com/WordPress/gutenberg/blob/23f11a18726bf69717595451ab3250cb67b426f6/packages/block-library/src/avatar/user-control.js#L11 https://github.com/WordPress/gutenberg/blob/08cf4a96d1f14fcdf4c55886ba237fb294f9c465/packages/block-library/src/query/edit/inspector-controls/author-control.js#L15 https://github.com/WordPress/gutenberg/blob/760815a731805fa05f6cbb234660b254ad1ca6c9/packages/block-library/src/post-author/edit.js#L43 https://github.com/WordPress/gutenberg/blob/711ea0eda4e3b4d28b9e0d70723ec04622dec278/packages/core-data/README.md?plain=1#L26

What is your proposed solution?

My suggestion would be to use the capability argument. The following code snippet fetches the edit_post capability for the current post type. This code is only compatible with WordPress 5.9. Either Gutenberg will need to increase the minimum supported version or include a check and a fallback with who: 'authors'.

const AUTHORS_QUERY = {
    per_page: 50,
    _fields: 'id,name',
    context: 'edit', // Allows non-admins to perform requests.
};
const currentPostType = wp.data.select('core/editor').getCurrentPostType();
const editPostCap = wp.data.select('core').getPostType( currentPostType ).capabilities.edit_post;
wp.data.select('core').getUsers( { ...AUTHORS_QUERY, capability: [ editPostCap ] } );

So that everyone does not have to figure out the edit_post capability, it might be best to create a helper function. Though in WordPress 5.9 the getAuthors function was deprecated https://github.com/WordPress/gutenberg/pull/33725

Related Issues: https://github.com/WordPress/gutenberg/issues/39035 & https://github.com/WordPress/gutenberg/issues/17364

[ocean90]

Noting that the current request already generates a X-WP-DeprecatedParam header for this:

X-WP-DeprecatedParam: WP_User_Query (since 5.9.0; <code>who</code> is deprecated. Use <code>capability</code> instead.)

@TimothyBJacobs @spacedmonkey @youknowriad Do you have some ideas how we can detect whether who or capability should be used?

[TimothyBJacobs]

I'm not sure what the progress is on the client having knowledge of the REST API schemas, cc @youknowriad. But the best way would be to look at the schema for the endpoint and check for the presence of the capability property in the GET args.

[grappler]

The only reason that I can see why we would need to check for the support is to be able to run the Gutenberg plugin on WordPress 5.8. Depending on the time it takes for a patch, WordPress 6.0 will have been released.

It might make more sense to increase the minimum required WP version. https://github.com/WordPress/gutenberg/blob/956a836ce8fcdd61b186cf1c2d1047b48126898d/gutenberg.php#L6

[ocean90]

As of #41306 the plugin now requires 5.9 so I guess we can switch the query to use capability now.