Prepare release notes for v2.1.0

Signed-off-by: Derek McGowan <derek@mcg.dev>

releases/v2.1.0.toml

@@ -9,7 +9,7 @@ ignore_deps = [ "github.com/containerd/containerd" ]
 # previous release
 previous = "v2.0.0"
 
-pre_release = true
+pre_release = false
 
 preface = """\
 The first minor release of containerd 2.x focuses on continued stability alongside

version/version.go

@@ -24,7 +24,7 @@ var (
 	Package = "github.com/containerd/containerd/v2"
 
 	// Version holds the complete version number. Filled in at linking time.
-	Version = "2.1.0-rc+unknown"
+	Version = "2.1.0+unknown"
 
 	// Revision is filled with the VCS (e.g. git) revision being used to build
 	// the program at linking time.