Expose confirmed at instead of confirmed

Confirmation time is more useful, if a user wants the boolean
they can use !confirmed_at.nil?

changelogs/unreleased/26327-include-whether-an-email-is-verified-in-the-user-emails-api-endpoin.yml

 ---
-title: Return whether an email is verified from email entity
+title: Return email confirmation time from email entity
 merge_request: 58957
 author:
 type: changed

doc/api/users.md

@@ -1214,12 +1214,12 @@ GET /user/emails
   {
     "id": 1,
     "email": "email@example.com",
-    "confirmed?" : true
+    "confirmed_at" : "2021-03-26T19:07:56.248Z"
   },
   {
     "id": 3,
     "email": "email2@example.com",
-    "confirmed?" : false
+    "confirmed_at" : null
   }
 ]
 ```
@@ -1260,7 +1260,7 @@ Parameters:
 {
   "id": 1,
   "email": "email@example.com",
-  "confirmed?" : true
+  "confirmed_at" : "2021-03-26T19:07:56.248Z"
 }
 ```
 
@@ -1280,7 +1280,7 @@ Parameters:
 {
   "id": 4,
   "email": "email@example.com",
-  "confirmed?" : true
+  "confirmed_at" : "2021-03-26T19:07:56.248Z"
 }
 ```
 

lib/api/entities/email.rb

@@ -3,7 +3,7 @@
 module API
   module Entities
     class Email < Grape::Entity
-      expose :id, :email, :confirmed?
+      expose :id, :email, :confirmed_at
     end
   end
 end

spec/requests/api/users_spec.rb

@@ -1795,7 +1795,7 @@ RSpec.describe API::Users do
         post api("/users/#{user.id}/emails", admin), params: email_attrs
       end.to change { user.emails.count }.by(1)
 
-      expect(json_response['confirmed?']).to be_falsey
+      expect(json_response['confirmed_at']).to be_nil
     end
 
     it "returns a 400 for invalid ID" do
@@ -1812,7 +1812,7 @@ RSpec.describe API::Users do
 
       expect(response).to have_gitlab_http_status(:created)
 
-      expect(json_response['confirmed?']).to be_truthy
+      expect(json_response['confirmed_at']).not_to be_nil
     end
   end