v2.1.0 planning

[AkihiroSuda]

Planning to release nerdctl v2.1 after:

• release of containerd v2.1 (tomorrow)
• unpacking needs flock AkihiroSuda/gomodjail#50
• feat: add support for userns #3941

cc @containerd/nerdctl-maintainers

[AkihiroSuda]

Will release after merging:

• update RootlessKit (2.3.5), fuse-overlayfs (1.15), containerd-fuse-overlayfs (2.1.5), etc. #4227

[AkihiroSuda]

😞

• CI: v2.1.0 binaries could not be released #4241