Centralize SECURITY.md using meta-repo {ORG}/.github

Github has a "community health file" feature: if you create the repo {ORG}/.github you can have a organization-wide SECURITY.md.

Example: as of 2026-06-17, jupyter/github-data has no SECURITY.md, but because SECURITY.md exists in jupyter/.github, the security policy is visible at https://github.com/jupyter/github-data/security/policy.

Documented in https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file.

**We should remove SECURITY.md from most repos in favor or community health files. This will centralize the documentation.**

Caveat: this is github-specific.