Bump golang in /trillian/examples/deployment/docker/ctfe

Bumps golang from 1.21.5-bookworm to 1.21.6-bookworm.

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

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

trillian/examples/deployment/docker/ctfe/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.21.5-bookworm@sha256:2d3b13c2a6368032e9697e64e7c923184e6e3be03cf01eadff27de124114e64e as build
+FROM golang:1.21.6-bookworm@sha256:b651ed8e3316720203ec18cbf8fa16a0f30d686b0ba5207f77ca5ae10d55da77 as build
 
 ARG GOFLAGS=""
 ENV GOFLAGS=$GOFLAGS