[docs] Document move from chromium to github to report security issues. (#95262)

I'm not aware of any other documentation changes that are needed to
complete the migration from chromium to github to report security
issues. The top-level security.md file refers to
https://llvm.org/docs/Security.html#how-to-report-a-security-issue for
documentation on reporting a security issue, which is being updated as
part of this PR.