Enable private vulnerability reporting once repo is public

[Variour]

GitHub's private vulnerability reporting (Settings → Code security and analysis → Private vulnerability reporting) is only available on public repositories, so it cannot be enabled while this repo is private.

Action items

• Enable private vulnerability reporting after repository visibility is switched to public
• Confirm SECURITY.md (Add SECURITY.md with a vulnerability disclosure process #339) still accurately describes the reporting process once enabled

Related: #339, #337