Modify DAST On Demand Scan CI Configuration to enable Browser Based Scan

Modify the DAST On Demand Scan CI Configuration to enable Browser Based Scan using the CI variable DAST_BROWSER_SCAN and setting it to true .

This CI variable should only be added to the generated CI configuration when the feature flag dast_ods_browser_based_scanner is enabled.

This will help transition On Demand Scans to start using Browser Based DAST.

Implementation Plan

1. Create a new development feature flag dast_ods_browser_based_scanner with default value false.
   1. https://docs.gitlab.com/ee/development/feature_flags/#development-type
   2. https://docs.gitlab.com/ee/development/feature_flags/#create-a-new-feature-flag
   3. Roll Out issue has already been created: #430499 (closed)
2. Modify ee/app/models/dast_scanner_profile.rb to append the CI variable DAST_BROWSER_SCAN when the feature flag is enabled

Related Epic: &11429 (closed)