canary: bump golang from 1.21.1 to 1.21.3 in /dependabot/docker/go

Bumps golang from 1.21.1 to 1.21.3.

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

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

dependabot/docker/go/Dockerfile

@@ -15,4 +15,4 @@
 # binaries) to reflect that version of Go.
 
 # https://hub.docker.com/_/golang
-FROM golang:1.21.1
+FROM golang:1.21.3