Bump golang from 1.21.4-bookworm to 1.21.5-bookworm in /integration

Bumps golang from 1.21.4-bookworm to 1.21.5-bookworm.

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

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

integration/Dockerfile

@@ -1,6 +1,6 @@
 # This Dockerfile builds a base image for the certificate-transparency-go CloudBuild integration testing.
 # See https://hub.docker.com/_/golang for the set of golang base images.
-FROM golang:1.21.4-bookworm@sha256:52362e252f452df17c24131b021bf2ebf1c9869f65c28f88ddb326191defea9c as ct_testbase
+FROM golang:1.21.5-bookworm@sha256:a6b787c7f9046e3fdaa97bca1f76fd23ff4108f612de885e1af87e0dccc02f99 as ct_testbase
 
 WORKDIR /testbase