Migrate govuk-visually-hidden to usa-sr-only in letter template

notifications_utils/jinja_templates/letter_image_template.jinja2

@@ -9,7 +9,7 @@
   </div>
 {% endfor %}
 
-<div class="govuk-visually-hidden">
+<div class="usa-sr-only">
   <h3>
     Recipient address
   </h3>

tests/notifications_utils/test_template_types.py

@@ -1200,7 +1200,7 @@ def test_letter_image_template_renders_visually_hidden_address():
         ),
         features="html.parser",
     )
-    assert str(template.select_one(".govuk-visually-hidden ul")) == (
+    assert str(template.select_one(".usa-sr-only ul")) == (
         "<ul>" "<li>line 1</li>" "<li>line 2</li>" "<li>postcode</li>" "</ul>"
     )