Bump golang in /examples/deployment/docker/log_signer

Bumps golang from `9c7ea4a` to `61f84bc`.

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

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

examples/deployment/docker/log_signer/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.21.1-bookworm@sha256:9c7ea4a4924ae24913401af45c9b6f439d0f782267584738d1cc1099d8b7a02c as build
+FROM golang:1.21.1-bookworm@sha256:61f84bc8cddb878258b2966d682c11a1317e97a371ff0da98823d9e326d9dac1 as build
 
 WORKDIR /trillian