Enable DAST site profiles FF by default

What does this MR do?

This enables the security_on_demand_scans_site_profiles_feature_flag feature flag by default based on the discussion in #235531 (comment 394262725)

We feel like we're ready to remove the feature flag altogether because the feature has been verified and seems to be working as expected. To be extra-safe, we're going to keep all the logic associated with this feature flag and enable it by default in %13.3, which would let us some margin to disable it later on. Early in %13.4, we'll remove the feature flag altogether.

How to test this?

• Enable the primary feature flag.

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

• Open the On-demand scans form at /:group/:project/-/on_demand_scans.

Screenshots

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry Even though this is removing a feature flag, the whole feature is actually hidden behind yet another feature flag, so this does not require a changelog entry.
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content