Update Security Dashboard layout for improved usability

What does this MR do?

This MR adds a few changes with the goal to improve UX of group security dashboard:

• Moves the vulnerabilities chart into an aside
• Hides vulnerabilities count
• Makes chart buttons span full width
• Adds date information to the chart
• Removes labels from charts
• Adds avg-max legend to chart

Screenshots

Before	After

Responsive behaviour

security_dashboard_after

Documentation Updates

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32205

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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 #12846 (closed)