Use "withdraw" instead of "retire"

We used "retire" because we translated it literally from the Spanish
verb "retirar" which can mean both "retire" and "withdraw".

Note we're still using "retire" in database fields and method names;
changing that might make it harder to upgrade from a previous version of
CONSUL.

spec/system/proposal_notifications_spec.rb

@@ -310,7 +310,7 @@ describe "Proposal Notifications" do
       expect(page).to have_content "This resource is not available anymore"
     end
 
-    scenario "Proposal retired by author" do
+    scenario "Proposal withdrawn by its author" do
       author = create(:user)
       user = create(:user)
       proposal = create(:proposal, :retired, author: author, followers: [user])