Add formatted-date-range component to usage_billing

What does this MR do and why?

Add <formatted-date-range /> component to usage_billing folder (will be used in upcoming MRs) and clean up the usage app !203275 (aef150c8).

note: the feature is behind a feature flag, it won't be enabled until the whole project is ready.

MR	status
Add usage billing app skeleton with its story (!203124 - merged)	✅
Add formatted-date-range component to usage_bil... (!203275 - closed)	this MR
Add purchase commitment card and use it in the ... (!203278 - merged)	⏳
Add usage trends chart to usage app (!203360 - merged)	⏳

References

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

• Ensure you're running GitLab as self-managed (e.g. 'export GITLAB_SIMULATE_SAAS=0 gdk restart`)
• Ensure you've a license added to your instance
• Enable billing_usage feature flag (e.g. http://gdk.test:3000/rails/features/usage_billing_dev)
• Navigate to the /admin/gitlab_duo/usage
• Verify the page is working without console errors

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #566204