Product analytics dashboards panel UI changes

What does this MR do and why?

Changes described in this MR comment

• Removes drop shadow
• increases the roundness of the border

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

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)

Related to #409947 (closed)