meta: fix links in `SECURITY.md`
Originally, this file linked to https://github.com/nodejs/nodejs.org, when it should link to https://github.com/nodejs/security-wg
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Originally, this file linked to https://github.com/nodejs/nodejs.org, when it should link to https://github.com/nodejs/security-wg