Make site_branding tag work with Django 4.0

A test for site_branding tag starts to fail with Django 4.0.
It seems to happen as settings.SITE_BRANDING is _("Horizon") and
a translation marker _() is no longer evaluated during rendering.

As a solution, this commit changes the implementation of
site_branding tag to use "simple_tag" method
as django.template.Library.simple_tag() [1] seems to handle
an i18n-ed string properly.

[1] https://docs.djangoproject.com/en/4.0/howto/custom-template-tags/#simple-tags

Closes-Bug: #1980214
Change-Id: I6fdfffbeef2b405da21289d37722e3f068e27fea

horizon/templatetags/branding.py

@@ -27,14 +27,9 @@ from django import template
 register = template.Library()
 
 
-class SiteBrandingNode(template.Node):
-    def render(self, context):
-        return settings.SITE_BRANDING
-
-
-@register.tag
-def site_branding(parser, token):
-    return SiteBrandingNode()
+@register.simple_tag
+def site_branding():
+    return settings.SITE_BRANDING
 
 
 @register.tag