Review the button's wording rule in the checkout flow

[ElodieOS]

Hi everyone!

In the checkout flow, the button's wording rule varies depending on the button. We have checked with @l-delin and we think it will be better to harmonize them with "Continue to [step name]" and an icon. It's better for guiding the user (except for the form validation buttons, which are there to validate the form and not to move on to the next step).