docs(SECURITY): sync security policy

[caugner]

Description

Syncs the SECURITY.md file with the canonical version from mdn/fred.

Motivation

Ensures consistent security reporting procedures across all MDN repositories.

Additional details

See: https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository

Related issues and pull requests

Part of https://github.com/mdn/fred/issues/1068.