Fix failing dashboard schema validation calls

What does this MR do?

The metrics dashboard (for custom dashboards) on page load performs a schema validation based on the dashboard file path. Now that we support Vanity URLs, custom dashboards can be loaded based on the file name.

However, the custom dashboard schema validation API hits the BE with dashboard filename instead of the file path causing the request to fail and surface an error.

This MR fixes the request sent to the schema validation API. If a dashboard schema is valid the warning message will not popup. Otherwise, it will.

Technical details

This MR fixes it by using the correct Vuex getter that has access to the entire dashboard path.

Screenshots

-	Before	After
Dashboard with invalid schema
Dashboard with valid schema

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

Mentions: #229613 (closed)