release note: add containerd v2.1 as a compatible version

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Author: AkihiroSuda

hack/generate-release-note.sh

@@ -25,7 +25,7 @@ cat <<-EOX
 (To be documented)
 
 ## Compatible containerd versions
-This release of nerdctl is expected to be used with containerd v1.6, v1.7, or v2.0.
+This release of nerdctl is expected to be used with containerd v1.6, v1.7, v2.0, or v2.1.
 
 ## About the binaries
 - Minimal (\`${minimal_amd64tgz_basename}\`): nerdctl only