Update letter pricing

The pricing page had the correct data, but there were a few pages where
the minimum and maximum price were out of date.

app/templates/views/features/letters.html

@@ -36,7 +36,7 @@
   <p>Read our <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.documentation') }}">documentation</a>.</p>
 
   <h2 class="heading heading-medium">Pricing</h2>
-  <p>It costs between 30p and 76p (plus VAT) to send a letter. Prices include:</p>
+  <p>It costs between 35p and 81p (plus VAT) to send a letter. Prices include:</p>
   <ul class="list list-bullet">
      <li>paper</li>
      <li>double-sided colour printing</li>

app/templates/views/service-settings/set-letter.html

@@ -17,7 +17,7 @@
         back_link=url_for('main.service_settings', service_id=current_service.id)
       ) }}
       <p>
-        It costs between 30p and 76p to send a letter using Notify.
+        It costs between 35p and 81p to send a letter using Notify.
       </p>
       <p>
         See <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for(".pricing", _anchor="letters") }}">pricing</a> for the list

tests/app/main/views/test_service_settings.py

@@ -3616,7 +3616,7 @@ def test_unknown_channel_404s(
 ), [
     (
         'letter',
-        'It costs between 30p and 76p to send a letter using Notify.',
+        'It costs between 35p and 81p to send a letter using Notify.',
         'Send letters',
         ['email', 'sms'],
         'False', 'True',
@@ -3624,7 +3624,7 @@ def test_unknown_channel_404s(
     ),
     (
         'letter',
-        'It costs between 30p and 76p to send a letter using Notify.',
+        'It costs between 35p and 81p to send a letter using Notify.',
         'Send letters',
         ['email', 'sms', 'letter'],
         'True', 'False',