docker: bump golang in /unstable/combined

Bumps golang from 1.21rc3-bookworm to 1.21rc4-bookworm.

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

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

unstable/combined/Dockerfile

@@ -7,7 +7,7 @@
 
 # https://hub.docker.com/_/golang
 
-FROM golang:1.21rc3-bookworm as builder
+FROM golang:1.21rc4-bookworm as builder
 
 # A current dev branch build (mirrored to fork) is used for pre-release Go
 # versions, otherwise the latest upstream build of the tool is installed in
@@ -56,7 +56,7 @@ RUN echo "Installing golangci-lint@${GOLANGCI_LINT_VERSION}" \
     && golangci-lint --version
 
 
-FROM golang:1.21rc3-bookworm as final
+FROM golang:1.21rc4-bookworm as final
 
 # 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"