VSA - Add info icons to explain lead time / cycle time calculations

What does this MR do?

Adds a description to the time metrics for VSA

• Adds a question mark next to Lead time and Cycle time
• Minor cleanup utility css classes

Testing instructions

1. Navigate to any group ie /gruops/gitlab-org
2. From the left side bar Navigate to Analytics > Value stream analytics
3. The Time panel should load, with a ? next to Lead time and Cycle time
4. Hover over the question mark to view a description

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #225452 (closed)