Median data is being requested unnecessarily for hidden stages

What does this MR do?

We noticed some extra requests being sent for some of the VSA data points, when a VSA stage is hidden we do not need to fetch the data to render in charts, instead we should ensure we only ever query data for stages that are active

• Ensures we only send requests for activeStages
• Uses the activeStages getter that's available on the VSA vuex state root
• Updates related specs

Testing

1. Browse to group VSA, for example /groups/gitlab-org/-/analytics/value_stream_analytics, or by navigating to a group and then selecting Analytics > Value Stream in the menu

2. In the network panel observe the requests being sent for /analytics/value_stream_analytics/stages/:stage_id/duration_chart and /analytics/value_stream_analytics/stages/:staged_id/median (filtering the requests will help)
3. Hide a stage, by hovering over a stage and clicking the 3 dots that appear

4. Click hide stage
5. Refresh the page and observe the requests

Screenshots

No visual changes

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

Related to #225660 (closed)