Bump golang from 1.20 to 1.21 in /cmd/exporter (#104)

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

cmd/exporter/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20 as builder
+FROM golang:1.21 as builder
 WORKDIR /workspace
 
 ARG GOPROXY