`classnames` Deprecation Warning

[jamesbiggs]

Issue Summary

When upgrading to wagtail 5.2, I get a RemovedInWagtail60Warning for the classnames attribute, telling me it'll be removed in wagtail 6.0, which comes from:

https://github.com/torchbox/wagtailmedia/blob/0573ddc9050e4adfb415cccdb725f19c1f63c3ea/src/wagtailmedia/wagtail_hooks.py#L73-L81

Steps to Reproduce

1. Upgrade to wagtail 5.2
2. See that you get a deprecation warning in the logs

Technical details

• Python version: 3.11.6
• Django version: 4.2.7
• Wagtail version: 5.2
• wagtailmedia version: 0.14.4
• Browser version: Chrome 118

[zerolab]

fixed via accd29a