Update button copy for On-demand scans Pages

Related issue - #354877 (closed)

What does this MR do and why?

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

Screenshots or screen recordings

Empty state

Scan List

New / Edit scan page

New	Edit scan

Please note the title contains the DAST keyword which will be removed in a follow-up MR.

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.