Removes unnecessary tabs from DAST view scans

What does this MR do and why?

This does a few changes to the on-demand DAST scans view app:

• We are removing the Running, Finished and Scheduled tabs. These were added as dummy components because the initial plan was to release all tabs in one go. We have since then decided to release the All tabs independently and to iteratively implement the other ones.
• We are passing the actual pipelines count to the app so that we don't rely on a hardcoded hasData property anymore. The app now has actual data to conditionally show the empty state or the tabs.
• We are renaming the itemCount prop to itemsCount, which sounds better semantically.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the on-demands scans page

   Feature.enable(:dast_view_scans)

2. Visit any project's on-demand scans page such as http://127.0.0.1:3000/groups/flightjs/-/on_demand_scans

If you have never run an on-demand scan before, you should see an empty state. From there, you can follow the instructions to run an on-demand scan. Once that is done, visit the on-demand scans page again and the All tab should show with the actual pipelines count.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #341364 (closed)