Metrics: Use a single source for the currently selected dashboard (in a Vuex getter)

What does this MR do?

Depends on: !31597 (merged)

Follow up from !31597 (comment 341476368)

Previously the currently selected dashboard was calculated on the fly by the dashboard component, but as other components needed this information, we added a getter (nullable) called selectedDashboard to calculate this information.

The selectedDashboard getter is used by the dashboard with this change.

Note: Also, the selected dashboard by default is not simply the first one, but the one marked default.

Screenshots

N/A

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

Closes #214581 (closed)