Enable dast_saved_scans feature flag by default

What does this MR do?

Enables the :dast_saved_scans feature flag by default.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required) #295618 (closed)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Related to #295252 (closed)