Promote malware filter feature flag to beta

What does this MR do and why?

The flag is fully enabled on GitLab.com, so promote it from wip to beta and default-enable it for self-managed. wip flags cannot be default-enabled, which is why the type has to change first.

References

Rollout issue: #600025 (closed)

Screenshots or screen recordings

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #600025 (closed)