Update billing account settings section

[gtsiolis]

Problem to solve

Following up from #13564 (comment) (Cc @jankeromnes), when users join and the UBP (Usage-based Pricing) is enabled for them, what they see under /billing can be confusing.

1. There's an unselected card which can be selected.
2. The footnote copy can be confusing and inaccurate, as we now support billing for personal accounts.
3. The footnote style is using a quite bold color, standing out from the rest of the page.

Default	Selected

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gtsiolis]

Closing this as it's no longer relevant.