Bump golang from 1.20 to 1.21 in /build (#947)

Bumps golang from 1.20 to 1.21.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

build/Dockerfile

@@ -1,5 +1,5 @@
 # syntax=docker/dockerfile:1.4
-FROM golang:1.20 as builder
+FROM golang:1.21 as builder
 ARG VERSION
 ARG GIT_COMMIT
 ARG DATE