Remove dependency_malware_detection feature flag

What does this MR do?

Removes the dependency_malware_detection feature flag. The enabled behavior is now unconditional: malware data is shown on dependency surfaces whenever the user can read security resources.

• Deletes the flag definition YAML.
• Removes the Group#dependency_malware_detection_feature_flag_enabled? and Project#dependency_malware_detection_feature_flag_enabled? helpers and all callers (REST API, GraphQL, serializers, project and group dependencies controllers).
• Frontend: stops reading glFeatures.dependencyMalwareDetection in the dependencies table, vulnerabilities popover, and the project and group filtered search. The popover now always shows the text that mentions malware packages.
• Removes specs (Ruby and Jest) that stub or toggle the flag, and the model specs for the helpers.
• Updates the dependency list docs and the GraphQL malware field description (no longer documents the null case when the flag is off).
• Regenerates public/-/graphql/introspection_result.json, doc/api/openapi/openapi_v3.yaml and locale/gitlab.pot for the changed descriptions and removed popover string.

Closes #629306 (closed)

Notes for reviewers

• Only merge once the flag is confirmed stable at GA, per the issue.
• The docs history entries say GitLab 19.5. Please adjust to the actual milestone.
• compile-test-assets is OOM-killed (exit 137) on the fork's runners. It is unrelated to this change and should pass in the canonical pipeline.