Copy changes for On-demand Scans Page

Proposal

This is to update the copy at https://gitlab.com/gitlab-org/gitlab/-/on_demand_scans

• Change button text from New DAST scan to New scan

Designs

#351476[On-demand_scans_-_all_scans.png]

Implementation Plan

• Update ee/app/assets/javascripts/on_demand_scans/components/on_demand_scans.vue
• Update ee/app/assets/javascripts/on_demand_scans/components/empty_state.vue
• Update corresponding specs