Bump golang in /examples/deployment/docker/log_signer

Bumps golang from 1.19-buster to 1.20-buster.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
...

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

examples/deployment/docker/log_signer/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.19-buster as build
+FROM golang:1.20-buster as build
 
 WORKDIR /trillian