Adopt classname instead of classnames in all MenuItem/Sidebar usage

[lb-]

In Wagtail 4.2 we started being more consistent with how we support classname kwargs in various components and templates, this seemed to be received well and means our code is more predictable.

At the time we avoided changing anything that was not yet fully documented or just had a small surface area. In this PR I propose we adjust one documented use case MenuItem (and related) with a clear deprecation path.

I think this will make using Wagtail more intuitive for developers and sets us up for easier customisation in the long term.

Details

• Adds a deprecation path, including documentation to further remove unpredictable naming of adding classname in Python APIs
• Intentionally keeps classNames in Telepath adaptor inner usage as this convention is not set, however, the server side value passed in will use classname
• See https://docs.wagtail.org/en/stable/contributing/general_guidelines.html#use-classname-in-python-html-template-tag-variables
• See previous rounds of clean up related to this - Ensure icon template tag can use classname instead of class_name by default #9769 & Ensure all icon template tags use classname instead of class_name & add deprecation warning #9770
• Avoiding touching the image template tag but if there is a desire, I can do that one as a future PR.

Checklist

• Do the tests still pass?
• Does the code comply with the style guide?
• For Python changes: Have you added tests to cover the new/fixed behaviour?
• For front-end changes: Did you test on all of Wagtail’s supported environments?
• For new features: Has the documentation been updated accordingly?

[squash-labs]

Manage this branch in Squash

Test this branch here: https://lb-cleanupadopt-classname-menu-9y7xb.squash.io

[zerolab]

Lotf of great stuff here, @lb- 👏🏼
Left a number of suggestions

[lb-]

Thanks @zerolab - ready for another round if you have time.

[zerolab]

This is so close. One minor suggestion.

Also 2 other points:

1. I know modeladmin is being deprecated, but it is worth updating it as well since ModelAdminMenuItem subclasses MenuItem and passes classnames in __init__ - https://github.com/wagtail/wagtail/blob/main/wagtail/contrib/modeladmin/menus.py#L24 (also L52)

2. One to address in a follow-up PR, I think - images.formats.Format that uses classnames as an arg

[lb-]

Thanks @zerolab fixed up now. Inc. modeladmin items.

[zerolab]

I just realised that we don't have tests for the new RemovedInWagtail60Warning

We have a number of them for other areas -
https://github.com/search?q=repo%3Awagtail%2Fwagtail+RemovedInWagtail60Warning+path%3A%2Ftests\%2F%2F&type=code - and I am torn.

Other than this, LGTM. Epic tidy up

[lb-]

OK I have add some tests for the core MenuItem & LinkMenuItem only (instead of all the other classes) if that's OK & push up with changelog added. If all good we can merge. Otherwise let me know if I should add deprecation warning tests for the full suite of all menu item related classes.

[zerolab]

[lb-]

Thanks @zerolab - I really appreciate the reviews.