Update DAST Authentication page to reflect correct limitations

What does this MR do and why?

Updates the DAST Authentication CI/CD variables table to remove the limitation of some variables being available only to on-demand DAST scans.

Reference: Slack thread (internal link)

Before

Some variables in the table were annotated with "Available to an on-demand proxy-based DAST scan."

After

Those variables are available in the browser-based DAST scanner as well so the annotation is no longer needed.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

please ignore the misplaced table header, it looks a like a bug in the screenshot function of Firefox. Looks fine actually.