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`

Dockerfile.rootless

@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1
 # Build frontend on the native platform to avoid QEMU-related issues with nodejs ecosystem
-FROM --platform=$BUILDPLATFORM docker.io/library/golang:1.26-alpine3.24 AS frontend-build
+FROM --platform=$BUILDPLATFORM docker.io/library/golang:1.27-alpine3.24 AS frontend-build
 RUN apk --no-cache add build-base git nodejs pnpm
 WORKDIR /src
 COPY package.json pnpm-lock.yaml pnpm-workspace.yaml ./
@@ -9,7 +9,7 @@ COPY --exclude=.git/ . .
 RUN make frontend
 
 # Build backend for each target platform
-FROM docker.io/library/golang:1.26-alpine3.24 AS build-env
+FROM docker.io/library/golang:1.27-alpine3.24 AS build-env
 
 ARG GITEA_VERSION
 ARG TAGS=""