chore: bump golang from 1.20-bullseye to 1.21-bullseye in /build/tooling

Bumps golang from 1.20-bullseye to 1.21-bullseye.

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

Signed-off-by: dependabot[bot] <support@github.com>

build/tooling/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20-bullseye@sha256:74b09b3b6fa5aa542df8ef974cb745eb477be72f6fcf821517fb410aff532b00
+FROM golang:1.21-bullseye@sha256:0ed263861f71db3c4fed4d900b621aca03a3b91b8bc5f1dd56584af191c89d68
 
 RUN GO111MODULE=on go install sigs.k8s.io/controller-tools/cmd/controller-gen@v0.10.0
 RUN GO111MODULE=on go install k8s.io/code-generator/cmd/conversion-gen@v0.25.4