json: use context json in go1.20

common/json/context_120.go

@@ -0,0 +1,23 @@
+//go:build !go1.21 && go1.20 && !without_contextjson
+
+package json
+
+import (
+	"github.com/sagernet/sing/common/json/internal/contextjson_120"
+)
+
+var (
+	Marshal    = json.Marshal
+	Unmarshal  = json.Unmarshal
+	NewEncoder = json.NewEncoder
+	NewDecoder = json.NewDecoder
+)
+
+type (
+	Encoder     = json.Encoder
+	Decoder     = json.Decoder
+	Token       = json.Token
+	Delim       = json.Delim
+	SyntaxError = json.SyntaxError
+	RawMessage  = json.RawMessage
+)

common/json/std.go

@@ -1,4 +1,4 @@
-//go:build !go1.21 || without_contextjson
+//go:build !go1.20 || without_contextjson
 
 package json