Matrix Image: Bump amd64/golang in /oldstable/combined

Bumps amd64/golang from 1.21.8-bookworm to 1.21.9-bookworm.

---
updated-dependencies:
- dependency-name: amd64/golang
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

oldstable/combined/Dockerfile

@@ -7,7 +7,7 @@
 
 # https://hub.docker.com/_/golang
 
-FROM amd64/golang:1.21.8-bookworm
+FROM amd64/golang:1.21.9-bookworm
 
 # https://docs.github.com/en/packages/learn-github-packages/connecting-a-repository-to-a-package
 LABEL org.opencontainers.image.source="https://github.com/atc0005/go-ci"