Remove sbom_scan_malware_findings feature flag

What does this MR do and why?

Removes the sbom_scan_malware_findings feature flag (beta, default_enabled: true). Matching SBOM components against malware advisories is now unconditional in the SBOM scan path.

• Delete the flag definition YAML.
• Remove malware_findings_enabled? and its guard in Gitlab::VulnerabilityScanning::SecurityReportBuilder.
• Remove the spec context that toggled the flag off.
• Update the dependency scanning docs history and remove the flag note.

Closes #629305 (closed)

Checklist

• Confirm the feature is stable at GA before merging
• Confirm the GA milestone in the docs history line (19.5 assumed)
• Verify no references to sbom_scan_malware_findings remain (git grep)