serve dlna: now only supported on go1.21 or later

This is due to use of go1.21 only constructs in github.com/anacrolix/log

cmd/serve/dlna/cds.go

@@ -1,3 +1,5 @@
+//go:build go1.21
+
 package dlna
 
 import (

cmd/serve/dlna/cms.go

@@ -1,3 +1,5 @@
+//go:build go1.21
+
 package dlna
 
 import (

cmd/serve/dlna/dlna.go

@@ -1,3 +1,5 @@
+//go:build go1.21
+
 // Package dlna provides DLNA server.
 package dlna
 

cmd/serve/dlna/dlna_test.go

@@ -1,3 +1,5 @@
+//go:build go1.21
+
 package dlna
 
 import (

cmd/serve/dlna/dlna_unsupported.go

@@ -0,0 +1,9 @@
+//go:build !go1.21
+
+// Package dlna is unsupported on this platform
+package dlna
+
+import "github.com/spf13/cobra"
+
+// Command definition is nil to show not implemented
+var Command *cobra.Command

cmd/serve/dlna/dlna_util.go

@@ -1,3 +1,5 @@
+//go:build go1.21
+
 package dlna
 
 import (

cmd/serve/dlna/mrrs.go

@@ -1,3 +1,5 @@
+//go:build go1.21
+
 package dlna
 
 import (