Add and apply Rails/WhereMissing rubocop rule

So now we know where to use the `where.missing` method which was
introduced in Rails 6.1.

Note this rule didn't detect all cases where the new method can be used.

app/models/concerns/sdg/relatable.rb

@@ -44,7 +44,7 @@ module SDG::Relatable
     end
 
     def pending_sdg_review
-      left_joins(:sdg_review).merge(SDG::Review.where(id: nil))
+      where.missing(:sdg_review)
     end
   end