[dependencies] Update golang Docker tag to v1.21

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20-alpine AS builder
+FROM golang:1.21-alpine AS builder
 LABEL maintainer="nekohasekai <[email protected]>"
 COPY . /go/src/github.com/sagernet/sing-box
 WORKDIR /go/src/github.com/sagernet/sing-box