Use sentence case for DORA metrics titles

What does this MR do and why?

Use sentence case for DORA metrics titles:

• Deployment Frequency → Deployment frequency
• Lead Time for Changes → Lead time for changes
• Time to Restore Service → Time to restore service
• Change Failure Rate → Change failure rate
• Critical Vulnerabilities over time → Critical vulnerabilities over time
• High Vulnerabilities over time → High vulnerabilities over time

MR acceptance checklist

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

Screenshots or screen recordings

(Note: these screenshots are from GDK with fake data)

Before	After	What changed?
		Titles in left column of table are in sentence case
		Single stat title above graph is now sentence case
		Single stat title above graph is now sentence case
		Single stat title above graph is now sentence case

How to set up and validate locally

1. Check out this branch locally
2. Visit a Value Streams Dashboard for a group in your gdk (http://gdk.test:3000/groups/$GROUP/-/analytics/dashboards/value_streams_dashboard)
3. You will see that the titles in the Metric column are now in sentence case.
4. Click through to "Deployment frequency". You will see that all instances of "Deployment frequency" are in sentence case.
5. Click through to "Lead time for changes". You will see that all instances of "Lead time for changes" are in sentence case.
6. Click through to "Time to restore service". You will see that all instances of "Time to restore service" are in sentence case.
7. Click through to "Change failure rate". You will see that all instances of "Change failure rate" are in sentence case.

Related to #442145 (closed) and #424397 (closed)