ISTWCMS-5667: make subtitle an <h2> for now, to match styling of title, which is an <h1>

We will have a follow-up to make these not be headers at all.

source/_patterns/04-components/site-slogan/site-slogan.twig

-<span class="uw-site-name__subtitle">{{ subtitle }}</span>
+<h2 class="uw-site-name__subtitle">{{ subtitle }}</h2>