Add canonical link for explore page

app/views/explore/projects/index.html.haml

 - @hide_top_links = true
 - page_title    _("Projects")
 - header_title  _("Projects"), dashboard_projects_path
+- page_canonical_link explore_projects_url
 
 = render_dashboard_gold_trial(current_user)
 

changelogs/unreleased/270420-fj-add-explore-canonical-link.yml

+---
+title: Add canonical link for default explore route
+merge_request: 45746
+author:
+type: changed

spec/features/dashboard/root_explore_spec.rb

@@ -30,4 +30,12 @@ RSpec.describe 'Root explore' do
 
     include_examples 'shows public projects'
   end
+
+  it 'includes canonical link to explore projects url' do
+    visit explore_root_path
+
+    canonial_link = page.find("head link[rel='canonical']", visible: false)
+
+    expect(canonial_link[:href]).to eq explore_projects_url
+  end
 end