Update balance element in billing page

[gtsiolis]

Problem to solve

Following up from #14594 and a recent chat with @svenefftinge, we'd like to simplify how we surface the available balance, monthly quota, and usage data on the billing page.

Proposal

To simplify the current state, we could move a way from showing the used credits to available credits, rename usage limit to monthly quota, etc.

BEFORE	AFTER