Consistent card titles for usage billing dashboard

What does this MR do and why?

Makes several UI polish changes to the card variants:

• GitLab Credits - Monthly committed pool
• GitLab Credits - One-Time Waiver
• On-demand credits used this billable month

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• yarn storybook:start
• Navigate to http://localhost:6006/?path=/story/ee-usage-quotas-usage-billing-app--default

MR acceptance checklist

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