feat(payment): INT-7503 add floating labels and change little css details for stripe link

[AlanMunozN]

What? INT-7503

Add floating labels behind an experiment

Why?

for StripeLink, we are looking to match the same styles used by BC

Testing / Proof

Customer BC / vs / LINK Shipping BC / vs / LINK

Merge at the same time

https://github.com/bigcommerce/checkout-js/pull/1202

@bigcommerce/checkout @bigcommerce/payments @bigcommerce/apex-integrations

[BC-ISilva]

💚 Tested!

QE Details:

PR implementation has been validated, following scenarios were tested:

1. Experiment “CHECKOUT-6879.enable_floating_labels“ OFF
2. Experiment “CHECKOUT-6879.enable_floating_labels“ ON
3. All Stripe elements, labels are rendered inside the corresponding text field
4. All Stripe elements, when the shopper starts typing, then labels. shall remain and float to the top of the field allowing for the entered text to be prominent.
5. Stripe Elements will inherit all style rules defined in Page Builder (Page Builder > Theme Styles > Checkout Page > Form Input Fields) so that the Stripe Elements match the style of the native BigCommerce elements.
6. Works across desktop and mobile browsers (Safari and FireFox browsers were used for validation).

Please review attached word file with validation details. INT-7503 - [Link] Support Floating Labels for Stripe Address Element.docx