Addons: use verbose_name instead of slug for sorting versions

[humitos]

In the response of /addons/ API endpoint we are using version.slug to generate addons.flyout.versions:

readthedocs.org/readthedocs/proxito/views/hosting.py

Lines 357 to 368 in a49f111

	"versions": [
	{
	# TODO: name this field "display_name"
	"slug": version_.slug,
	"url": resolver.resolve(
	project=project,
	version_slug=version_.slug,
	external=version_.type == EXTERNAL,
	),
	}
	for version_ in versions_active_built_not_hidden
	],

Note there aren't too many differences here, but a version named like release/v2.x (verbose_name) will be rendered in the flyout as release-v2.x instead (slug).

Also note that we could consider to render the versions completely (using the VersionSerializer) under adddons.flyout.versions instead of just { slug: ..., url: ... }

Reference #11069 (comment)