Use <main> in SDG index

Just like we do in the other sections.

We're also moving the banner inside the <main> element because we do it
the same way in other sections.

app/components/sdg/goals/index_component.html.erb

@@ -1,8 +1,8 @@
 <% provide(:title) { title } %>
 
-<%= render Shared::BannerComponent.new("sdg") %>
+<main class="sdg-goals-index">
+  <%= render Shared::BannerComponent.new("sdg") %>
 
-<div class="sdg-goals-index">
   <%= link_list(*goal_links, class: "sdg-goal-list") %>
 
   <% phases.each do |phase| %>
@@ -14,4 +14,4 @@
       <%= render "shared/cards", cards: phase.cards %>
     </section>
   <% end %>
-</div>
+</main>