Bump golang in /integration/cloudbuild/testbase

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>

integration/cloudbuild/testbase/Dockerfile

@@ -1,5 +1,5 @@
 # This Dockerfile builds a base image for Trillan integration tests.
-FROM golang:1.21.1-bookworm@sha256:9c7ea4a4924ae24913401af45c9b6f439d0f782267584738d1cc1099d8b7a02c
+FROM golang:1.21.1-bookworm@sha256:61f84bc8cddb878258b2966d682c11a1317e97a371ff0da98823d9e326d9dac1
 
 WORKDIR /testbase