Fix WP 5.9 check for conditionally running code

[Mamaduka]

Description

Use a different function to check if the plugin runs on an older WP version. The wp_is_block_theme got ported back to the plugin (#37218). PR update conditional check to use a function that's WP core specific and was introduced in 5.9 - wp_list_users

The current logic only works because the compat/wordpress-5.9/theme.php file is included later in load.php.

How has this been tested?

1. Make sure you're running WordPress 5.9
2. Change something related to one of the post types in the Gutenberg codebase (i.e., one of the labels)
3. Load the relevant part of the UI and notice the change is not displayed because this code isn't active.

Borrowed Copied from #36898.

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[talldan]

Thanks. Looks like this code went on a journey, wp_is_block_theme isn't even what I originally added here 😄

[talldan]

I don't think it needs backporting, but might be good to keep the wp/trunk branch in sync like the other PRs.