Move approval_policies_block_on_failed_scan_job flag to beta and default enabled

What does this MR do and why?

Move the approval_policies_block_on_failed_scan_job feature flag from gitlab_com_derisk to beta, and set default_enabled to true.

The flag was rolled out in production about one week ago .

References

#604648 (closed)

Rollout issue: #604861 (closed) Clean up issue: #627271

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