chore: update Go to v1.27 (#39068)

Only made some necessary changes:

1. remove `GOEXPERIMENT`, only use jsonv2
1. `make fmt`
* `SigningKey` and `Signature` were affected due to some bugs in the
toolchain, so rewrote them
1. remove or fix fragile magic numbers and strings
    * the outputs of image/gzip/zlib packages are different
1. update "nolint" comments for the changed lint behaviors
1. add `tls.MLKEM1024`

modules/json/jsonlegacy.go

@@ -1,25 +0,0 @@
-// Copyright 2025 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-//go:build !goexperiment.jsonv2
-
-package json
-
-import (
-	"encoding/json" //nolint:depguard // this package wraps it
-	"io"
-)
-
-func getDefaultJSONHandler() Interface {
-	return jsonV1{}
-}
-
-func MarshalKeepOptionalEmpty(v any) ([]byte, error) {
-	return DefaultJSONHandler.Marshal(v)
-}
-
-func NewDecoderCaseInsensitive(reader io.Reader) Decoder {
-	return DefaultJSONHandler.NewDecoder(reader)
-}
-
-type Value = json.RawMessage