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/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