Complete security policy with mentions of remote code (#29707)

* Security policy

* Apply suggestions from code review

Co-authored-by: Luc Georges <McPatate@users.noreply.github.com>
Co-authored-by: Michelle Habonneau <83347449+Michellehbn@users.noreply.github.com>

* Update SECURITY.md

Co-authored-by: Diogo Teles Sant'Anna <diogoteles@google.com>

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Co-authored-by: Luc Georges <McPatate@users.noreply.github.com>
Co-authored-by: Michelle Habonneau <83347449+Michellehbn@users.noreply.github.com>
Co-authored-by: Diogo Teles Sant'Anna <diogoteles@google.com>