Prepare for v0.12.1

[ktock]

This release publishes a container image usable as a Kind node to `ghcr.io/containerd/stargz-snapshotter:0.12.1-kind`

$ kind create cluster --name stargz-demo --image ghcr.io/containerd/stargz-snapshotter:0.12.1-kind

kind >= v0.16.0 is recommended as the image is based on `kindest/node:v1.25.2` supported by kind v0.16.0.
See also kind release note: https://github.com/kubernetes-sigs/kind/releases/tag/v0.16.0

## Notable Changes

- Allow manually remove invalid snapshots on restore (#901)
- ctr-remote: Allow analyzer waiting for a line from the container (#933)
- Build and push stargz-snapshotter image usable as a kind node (#940)