Bump golang from 1.21.3 to 1.21.4 (#132)

Bumps golang from 1.21.3 to 1.21.4.

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

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

Dockerfile

@@ -1,5 +1,5 @@
 ############# builder
-FROM golang:1.21.3 AS builder
+FROM golang:1.21.4 AS builder
 
 WORKDIR /go/src/github.com/gardener/gardener-extension-os-gardenlinux
 COPY . .