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

modules/json/jsonv1.go

@@ -1,38 +0,0 @@
-// Copyright 2025 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-package json
-
-import (
-	"bytes"
-	"encoding/json" //nolint:depguard // this package wraps it
-	"io"
-)
-
-type jsonV1 struct{}
-
-var _ Interface = jsonV1{}
-
-func (jsonV1) Marshal(v any) ([]byte, error) {
-	return json.Marshal(v)
-}
-
-func (jsonV1) MarshalWrite(w io.Writer, v any) error {
-	return json.NewEncoder(w).Encode(v)
-}
-
-func (jsonV1) Unmarshal(data []byte, v any) error {
-	return json.Unmarshal(data, v)
-}
-
-func (jsonV1) NewEncoder(writer io.Writer) Encoder {
-	return json.NewEncoder(writer)
-}
-
-func (jsonV1) NewDecoder(reader io.Reader) Decoder {
-	return json.NewDecoder(reader)
-}
-
-func (jsonV1) Indent(dst *bytes.Buffer, src []byte, prefix, indent string) error {
-	return json.Indent(dst, src, prefix, indent)
-}

modules/json/jsonv2.go

@@ -1,8 +1,6 @@
 // Copyright 2025 The Gitea Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
-//go:build goexperiment.jsonv2
-
 package json
 
 import (
@@ -13,8 +11,6 @@ import (
 	"io"
 )
 
-// JSONv2 implements Interface via encoding/json/v2
-// Requires GOEXPERIMENT=jsonv2 to be set at build time
 type JSONv2 struct {
 	marshalOptions                  jsonv2.Options
 	marshalKeepOptionalEmptyOptions jsonv2.Options