[BNPL] Adjust payment method icons on the interface

[a-danae]

We're basing our designs on WooPayments. The payment method icons we introduced for BNPL have a padding around them, while they don't have it in WooPayments.

Designs	Current

Describe the solution you'd like

Adjust our icons and their styling to match how they look in WooPayments, at least for the newly introduced Klarna, Afterpay, and Affirm.

Additional context

Mentioned by @james-allan in https://github.com/woocommerce/woocommerce-gateway-stripe/pull/3040#pullrequestreview-2018712355

[a-danae]

Decided to go for a fix specific for Klarna instead of unifying the styles for all of our icons. That has a lower priority as of now.