Change classnames -> classname in wagtail_hooks.py

[jamesbiggs]

This PR was made to satisfy the issue: #224

Updated the wagtail_hooks.py that contains a hook using the classnames attribute which is bringing in a RemovedInWagtail60Warning when upgrading to wagtail 5.2.

Changing this to use classname will resolve the deprecation warning.

[jamesbiggs]

Hey @zerolab, I thought this'd be another easy one - but not sure why the CI is failing? Is it because it's not compatible with earlier versions or something?

[zerolab]

@jamesbiggs there was a typo in one of the tests in main 🙈

At the same time, classname only came in effect in 4.2, so this would be broken for 4.1

I've switched to using icon_name like everywhere else in Wagtail. And kept your commit. Thank you