Use future dates in has_multiple_dates

Only dates in the future are shown in the consent request email, so we
should update `has_multiple_dates` to work in the same way so the
content is consistent between the dates shown and whether to include a
sentence about multiple dates.

Jira-Issue: MAV-1277

Author: thomasleese

app/lib/govuk_notify_personalisation.rb

@@ -162,11 +162,7 @@ def full_and_preferred_patient_name
   def has_multiple_dates
     return nil if session.nil?
 
-    if session.today_or_future_dates.length > 1
-      "yes"
-    else
-      "no"
-    end
+    session.future_dates.length > 1 ? "yes" : "no"
   end
 
   def host

spec/lib/govuk_notify_personalisation_spec.rb

@@ -104,6 +104,7 @@
 
     it "doesn't show today's date in next date" do
       expect(to_h).to include(
+        has_multiple_dates: "no",
         next_or_today_session_date: Date.current.to_fs(:short_day_of_week),
         next_session_date: Date.tomorrow.to_fs(:short_day_of_week)
       )