wagtail-modeladmin is deprecated

[kesara]

          > I see some major version upgrades, in particular, `wagtail-modeladmin`.

For wagtail-modeladmin, the changelog says that the breaking changes are dropping old versions of Django and Wagtail, but the versions in use here are still supported.

However, wagtail-modeladmin is deprecated, and the recommended upgrade path is to use snippets instead. There is only one use in the codebase, which should be an easy conversion to a snippet:

www/ietf/utils/wagtail_hooks.py

Lines 31 to 42 in b222992

	class MenuItemAdmin(OrderableMixin, ModelAdmin): # type: ignore
	model = SecondaryMenuItem
	menu_order = 900
	menu_label = "Secondary Menu"
	menu_icon = "list-ul"
	add_to_settings_menu = True
	list_display = ("title",)
	ordering = ["sort_order"]
	modeladmin_register(MenuItemAdmin)

Originally posted by @mgax in #483 (comment)