v1.21.0

• New support for creating "simple signing" signatures using Sequoia-PGP, dependent on a build tag that enables it
• New option skopeo copy --force-compression-format
• New option --user-agent-prefix
• TLS options on the command line of skopeo sync take precedence over options in YAML

This is intended to match dependencies of Podman 5.7 as closely as possible.