Fix classname deprecation warning in Wagtail 4.2

[chosak]

Wagtail 4.2 deprecates use of the class_name attribute in the icon tag.
This commit replaces it with classname instead. See:

https://docs.wagtail.org/en/stable/releases/4.2.html#adoption-of-classname-convention-for-some-template-tags-includes

[chosak]

The Wagtail 4.2 tests are failing here for the same reason as cfpb/wagtail-sharing#66; @willbarton should we bring over something similar to cfpb/wagtail-sharing#68 here?

[willbarton]

@chosak Hmm. I think my preference would be to only test against LTS + latest supported version for Wagtail, because of the way their release cycle goes. This means 4.2 would be unnecessary to test against. It's a shame that release is broken, and unlikely to be fixed.

Wagtail-Flags doesn't already have its own separate test app, and it doesn't really need one, so we'd be adding one just to work around a problem upstream in an unsupported version of Wagtail.

I think my preference here is, if we need to test against 4.2, let's pin the last good release of 4.2 until we get to the 5.x LTS release as latest.

[willbarton]

Actually — does Wagtail-Flags use anything from the test app in its tests? I don't see anything.

It's possible we copied boilerplate settings for tests that include it from other apps that need it.

[chosak]

Good point! I can open another PR to remove dependence on the testapp.

I do have another question, though, about this PR: if/when we merge this in, it'll mean this package is no longer compatible with Wagtail 4.1 and below. So the next release could be 4.2+ only. Or we could hold off merging this until such time as we want to drop support for those older versions; after all, this is only a deprecation warning, not broken functionality. What do you think?

[willbarton]

Or we could hold off merging this until such time as we want to drop support for those older versions; after all, this is only a deprecation warning, not broken functionality. What do you think?

I think let's merge, but hold off on releasing until we're able to support the next LTS?