Update github.com/containerd/containerd to v1.5.7

... to include a fix for
https://github.com/advisories/GHSA-c2h3-6mxw-7mvq .

(Note that Skopeo doesn't depend on the vulnerable code,
so this is primarily to avoid dependency checker warnings.)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/google.golang.org/grpc/internal/credentials/util.go

@@ -18,7 +18,9 @@
 
 package credentials
 
-import "crypto/tls"
+import (
+	"crypto/tls"
+)
 
 const alpnProtoStrH2 = "h2"