Update Usage Billing Card Copy

What does this MR do and why?

This MR updates the copy on the Usage Billing cards based on feedback from the Technical Writing team to improve clarity and consistency:

• GitLab Credits cards: Add prepositions for clarity in "Used in this billing period"
• Monthly committed pool: Use "used" instead of "utilized" and align credit terminology in the breakdown
• One-Time Waiver: Replace "any" with "all" for accuracy since these credits are used after all other available credits are exhausted
• Increase monthly credit commitment: Make the discount messaging more specific and informative

References

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/578547

Screenshots or screen recordings

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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