Answer the OpenSSF Best Practices criteria that asked for policies the
project follows but had not written down:
- SECURITY.md: a first response within 14 days, publishing an advisory
with credit once a fix is released, and that only the latest release
receives security fixes.
- Governance: who has access to the project's resources, how write or
admin access is granted, and how CI secrets are stored and rotated.
- Quality assurance: how dependencies of the build, test, and
documentation tooling are pinned, scanned, and kept free of known
vulnerabilities.
Also update the assurance case, since comparison no longer recurses per
nesting level (#5390), and point the best practices badge and links to
bestpractices.dev under the program's current name.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>