Rename unprocessable_entity to unprocessable_content

When the 422 status was initially proposed in
[RFC4918](https://datatracker.ietf.org/doc/html/rfc4918) (2007), it was
called 'Unprocessable entity'.

When it became a standard in
[RFC9110](https://datatracker.ietf.org/doc/html/rfc9110) (2022), it was
renamed 'Unprocessable content'.

Rack now supports it as `:unprocessable_content` and has deprecated
`:unprocessable_entity`. This commit updates it to use the new name.

Co-authored-by: David Biddle <david.biddle@digital.cabinet-office.gov.uk>

Author: lfdebrux

app/controllers/account/contact_for_research_controller.rb

@@ -14,7 +14,7 @@ def update
       if @contact_for_research_input.submit
         redirect_to next_path
       else
-        render :edit, status: :unprocessable_entity
+        render :edit, status: :unprocessable_content
       end
     end
 

spec/controllers/forms/copy_of_answers_controller_spec.rb

@@ -94,9 +94,9 @@
     context "when the send_copy_of_answers value is invalid" do
       let(:send_copy_of_answers) { "invalid_value" }
 
-      it "renders the new template with unprocessable entity status" do
+      it "renders the new template with unprocessable content status" do
         post(copy_of_answers_create_path(form_id: form.id), params:)
-        expect(response).to have_http_status(:unprocessable_entity)
+        expect(response).to have_http_status(:unprocessable_content)
         expect(response).to have_rendered :new
         expect(response.body).to include("Sorry, there was a problem. Please try again.")
       end