Bump golang from 1.20-alpine to 1.21-alpine in /build (#4229)

Bumps golang from 1.20-alpine to 1.21-alpine.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
(cherry picked from commit b8aaeea)

Author: dependabot[bot]

build/Dockerfile

@@ -254,7 +254,7 @@ LABEL org.nginx.kic.image.build.nginx.version="${NGINX_PLUS_VERSION}${NGINX_VERS
 
 
 ############################################# Build nginx-ingress in golang container #############################################
-FROM golang:1.20-alpine AS builder
+FROM golang:1.21-alpine AS builder
 ARG IC_VERSION
 ARG TARGETARCH