MDL-85504 form: Legend's label template data should not be escaped

The label context data in the fieldset's legend tag should not be
escaped and allow raw HTML given that legend tags can contain
phrasing content and headings.

See developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/legend.

public/lib/form/templates/element-group.mustache

@@ -9,7 +9,7 @@
     {{$element}}
         {{#label}}
             <fieldset class="w-100 m-0 p-0 border-0">
-                <legend class="visually-hidden">{{label}}</legend>
+                <legend class="visually-hidden">{{{label}}}</legend>
                 <div class="d-flex flex-wrap align-items-center">
                 {{#element.elements}}
                     {{{separator}}}