Enable `dast_site_validation_drawer` feature flag by default

Feature flag rollout issue: #387619 (closed)

Epic: Add support for site validation from Drawer

What does this MR do and why?

This enables the flag (dast_site_validation_drawer) by default as it works as expected and seems stable after enabling this on .com

Screenshots or screen recordings

dast_site_validation_in_drawer

How to set up and validate locally

1. Visit https://gitlab.com/gitlab-org/secure/dast/dast-testing/-/on_demand_scans/new
2. Create a new site profile
3. Validate the profile
4. The new UX allows you to validate profile right from the drawer.

MR acceptance checklist

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