use experimental go json v2 library (#35392)

details: https://pkg.go.dev/encoding/json/v2

---------

Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/json/jsonlegacy.go

@@ -0,0 +1,24 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !goexperiment.jsonv2
+
+package json
+
+import (
+	"io"
+
+	jsoniter "github.com/json-iterator/go"
+)
+
+func getDefaultJSONHandler() Interface {
+	return JSONiter{jsoniter.ConfigCompatibleWithStandardLibrary}
+}
+
+func MarshalKeepOptionalEmpty(v any) ([]byte, error) {
+	return DefaultJSONHandler.Marshal(v)
+}
+
+func NewDecoderCaseInsensitive(reader io.Reader) Decoder {
+	return DefaultJSONHandler.NewDecoder(reader)
+}