Swap main with article-tag in sec dashboard app

This commit replaces the currently used `<main>` with an
`<article>` to ensure component re-usability.

(Using a main tag within a vue-component introduces the possibility
of having two or more main tags on a page, this causes accessbility
issues)

ee/app/assets/javascripts/security_dashboard/components/app.vue

@@ -129,12 +129,12 @@ export default {
     <vulnerability-count-list v-if="isLockedToProject" class="mb-0" />
 
     <div class="row mt-4">
-      <main role="main" class="col" :class="{ 'col-xl-7': !isLockedToProject }">
+      <article class="col" :class="{ 'col-xl-7': !isLockedToProject }">
         <security-dashboard-table
           :dashboard-documentation="dashboardDocumentation"
           :empty-state-svg-path="emptyStateSvgPath"
         />
-      </main>
+      </article>
 
       <aside v-if="!isLockedToProject" class="col-xl-5">
         <vulnerability-chart />

ee/spec/features/groups/group_overview_spec.rb

@@ -40,9 +40,7 @@ describe 'Group overview', :js do
       it 'displays the Security Dashboard view' do
         visit group_path(group)
 
-        page.within(find('main')) do
-          expect(page).to have_selector('.js-security-dashboard-table')
-        end
+        expect(page).to have_selector('.js-security-dashboard-table')
 
         page.within(find('aside')) do
           expect(page).to have_content 'Vulnerabilities over time'