Analytics dashboards UI polish

What does this MR do and why?

Applies slight UI improvements to the analytics dashboards

Summary:

• Reduce padding on title and description
• Remove horizontal line between title and dashboard content
• Change background color to gray-10
• Add small shadow to dashboard panels

Screenshots or screen recordings

Before	After

How to set up and validate locally

Feature flags

• Feature.enable(:group_analytics_dashboards): enables the group dashboard list
• Feature.enable(:group_analytics_dashboard_dynamic_vsd): renders the dashboard using shared analytics dashboards

Viewing the page

• View the VSD for any group (ex. http://gdk.test:3000/groups/gitlab-org/-/analytics/dashboards/value_streams_dashboard)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #409947 (closed)