Update metrics dashboard finder to list all predefined dashboards

What does this MR do?

Currently, any new out of the box dashboard has to be manually added to the Gitlab::Metrics::Dashboard::Finder file so that it is included in the list of all dashboards on the metrics dashboard page.

This MR updates Gitlab::Metrics::Dashboard::Finder so that all out of the box dashboards get automatically listed, and a manual change has to be done to exclude a dashboard.

Any new out of the box dashboards will still have to be added to the spec/lib/gitlab/metrics/dashboard/finder_spec.rb spec file, so we won't have new dashboards unexpectedly appearing in the list. The benefit of this change is that, the addition of a new out of the box dashboard will result in a spec failure, and the author will have to choose whether to allow the new dashboard to be listed or exclude it. Without this change, it is easy to add a new dashboard and forget to add it to the list of all dashboards.

Issue: #218649 (closed)

Screenshots

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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