Rollout of FF `security_on_demand_scans_site_profiles_feature_flag`

• Rollout issue: #233110 (closed)

What does this MR do?

This MR rolls out the security_on_demand_scans_site_profiles_feature_flag feature flag.

As we iterate on the On-demand DAST Scans feature, we're planning on introducing a slightly different form for creating such pipelines. To be on the safe side, we want to preserve the current form for now while we develop its next iteration behind a feature flag.

Rather than supporting the feature flag in the already existing component, this MR simply duplicates it so that we can render either the new or the old component depending on whether the feature flag is enabled or not. This approach will make it very easy to remove the feature flag once we're ready to do so, as we'll be able to simply delete the old files and drop the condition in the root component.

How to test this?

1. Make sure the main feature flag is already enabled. Yes, we're nesting feature flags, we're that crazy 🙃

echo "Feature.enable(:security_on_demand_scans_feature_flag)" | spring rails c

2. Visit a project's on-demand scan form: /:group/:project/-/on_demand_scans/.
3. Click on Create new DAST scan.
4. What you're seeing is the old form.
5. Enable the new feature flag:

echo "Feature.enable(:security_on_demand_scans_site_profiles_feature_flag)" | spring rails c

6. Repeat steps 2 & 3.
7. What you're seeing now is the new form. It looks the same as the other one, but I promise it's not 🤞

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry this does not require a changelog entry as this introduces a feature flag.
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #233110 (closed)