Only load active payment methods in the backend

This was the original behaviour in Solidus < 2.1, but the active scope
got lost in solidusio#1540. Payment methods used to use the `available` filter
which was scoped to active payment methods only. The `available_to_admin`
scope isn't limited active payment methods, so we need to add it here.

backend/app/controllers/spree/admin/payments_controller.rb

@@ -78,7 +78,7 @@ def object_params
 
       def load_data
         @amount = params[:amount] || load_order.total
-        @payment_methods = Spree::PaymentMethod.available_to_admin
+        @payment_methods = Spree::PaymentMethod.active.available_to_admin
         if @payment && @payment.payment_method
           @payment_method = @payment.payment_method
         else

backend/spec/controllers/spree/admin/payments_controller_spec.rb

@@ -92,7 +92,7 @@ module Admin
               @payment_method = create(:check_payment_method, available_to_admin: true, active: false)
             end
 
-            it "does not load the payment method", :pending do
+            it "does not load the payment method" do
               get :new, params: { order_id: order.number }
               expect(response.status).to eq(200)
               expect(assigns[:payment_methods]).to be_empty