Bump golang from 1.20 to 1.21 in /build

Bumps golang from 1.20 to 1.21.

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

Signed-off-by: dependabot[bot] <[email protected]>

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