Bump golang from 1.21.5 to 1.21.6 in /cmd/ruler (#145)

Bumps golang from 1.21.5 to 1.21.6.

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

cmd/ruler/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.21.5 as builder
+FROM golang:1.21.6 as builder
 WORKDIR /workspace
 
 ARG GOPROXY