Checkout progress links are not adequately labeled for screen readers

Describe your bug or feature request.

Example: We are on /checkout/x/order_information, so the previous checkout steps link is "Login" (translated from German, I hope its right). The Screenreader reads "Login". So this is very confusing. It should be "Back to checkout step login" or something like that.

Its probably a bad idea to add an additional aria-label, I thinks its best to solve it like this:

  <span class="checkout-progress__step-label">
       {# step.label content: #}
       <a href="/checkout/68100/login"><span class="visually-hidden">Back to step:</span> Login</a>
   </span>

If a bug, provide steps to reproduce it from a clean install.

1. https://commerce.demo.centarro.io/products/dining-armchair-pink => add to cart
2. Go to checkout
3. Continue as guest
4. Go to step 2 and check the link label