Release DAST UI additional variables feature

What does this MR do and why?

Feature Epic: &16057 (closed)

1. This MR removes the dast_ui_additional_variables feature flag, enabling DAST variable configuration in the UI for all users.
2. Documentation changes

References

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MR acceptance checklist

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Before	After

How to set up and validate locally

1. Go to a projects DAST profile library: -/security/configuration/profile_library#site-profiles
2. Create a new Site Profile
3. Configure variables under the Additional variables section
4. Save the profile
5. Run the DAST scan, the variables should be configured correctly.

Numbered steps to set up and validate the change are strongly suggested.