Fix query timeout on instance security dashboard

The query we were using to fetch the list of vulnerability findings for the instance security dashboard was fetching vulnerabilities for all pipelines and then filtering them by project.

Now we filter the pipelines we're fetching by project, greatly reducing the time to execute the query.

There are some additional bonuses to this change:

• removes some exception logic from Gitlab::Vulnerabilities::History
• simplifies Security::VulnerabilityFindingsController
• simplifies specs by making the Vulnerable for the instance security dashboard a better duck type with Group
• renames ApplicationInstance to InstanceSecurityDashboard, since its new logic is more specific. The new name isn't perfect since it doesn't really represent a model level domain concept, but at least it's clear what it's being used for.

#197238 (closed)

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

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