Bump golang in /integration/cloudbuild/testbase

Bumps golang from 1.20.8-bookworm to 1.21.1-bookworm.

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

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:1.20.8-bookworm@sha256:b099af7250d673d249597b79049e3e53487c435acf285e07a36d4b49f9f4750a
+FROM golang:1.21.1-bookworm@sha256:d3114db136e7f2d9b08c3fcd374381ba3f8d85c17b80c48dbc53cd938cfb2b64
 
 WORKDIR /testbase