MDL-80121 gradereport_singleview: Increase zero-state heading level

With the h2 heading not being rendered on the zero-state pages of the
Single view report, we need to increase the heading level of the
"Search for a user to view all their grades" and
"Select a grade item above" headings from h3 to h2.

grade/report/singleview/templates/zero_state_grade.mustache

@@ -26,6 +26,6 @@
 }}
 <div class="zero-state mx-auto w-50 text-center my-6">
     <img src="{{imglink}}" alt="{{#str}}viewsinglegradeitem, gradereport_singleview{{/str}}" aria-hidden="true" class="my-5">
-    <h3>{{#str}}viewsinglegradeitem, gradereport_singleview{{/str}}</h3>
+    <h2 class="h3">{{#str}}viewsinglegradeitem, gradereport_singleview{{/str}}</h2>
     <p>{{#str}}singleviewdescription, gradereport_singleview{{/str}}</p>
 </div>

grade/report/singleview/templates/zero_state_user.mustache

@@ -26,6 +26,6 @@
 }}
 <div class="zero-state mx-auto w-50 text-center my-6">
     <img src="{{imglink}}" alt="{{#str}}viewsingleuser, gradereport_singleview{{/str}}" aria-hidden="true" class="my-5">
-    <h3>{{#str}}viewsingleuser, gradereport_singleview{{/str}}</h3>
+    <h2 class="h3">{{#str}}viewsingleuser, gradereport_singleview{{/str}}</h2>
     <p>{{#str}}singleviewdescription, gradereport_singleview{{/str}}</p>
 </div>