Add page title and description to Security dashboard

Problem to solve

Customers are confused by the criteria that goes into vulnerability data reflected on the project-level security dashboard. (See Security Dashboard should not show vulnerabilit... (#440837 - closed).) We don't offer filters on the dashboard widget, and don't specify the criteria of the vulns shown in the panel below.

More reasons to support this proposal:

• This is one of the few pages that doesn't have a page title and description. The Vulnerability report, Dependency list, and Policies pages all have page titles and descriptions.
• The Vulnerability Report (as we know it today) used to be titled the Security Dashboard. I think adding titles to the Security Dashboard will clarify to customers and CSMs which is which now when they're making requests.
• Page titles and descriptions are important from an accessibility POV; assistive technologies (such as screen readers) rely on the page being labeled properly. From Pajamas:

Every page should have a level 1 (h1) heading. It can be visually hidden if the context of the page is clear for all users, but it must be present in the DOM.

Current state:

Proposal

• Add page title and description to project-level security dashboard. Link goes to the Security dashboard docs: https://docs.gitlab.com/ee/user/application_security/security_dashboard/#security-dashboards
• Changes x-axis label of Time to Date, and in the popover.