Bump golang from 1.18.3 to 1.21.4 in /injector (#32)

Bumps golang from 1.18.3 to 1.21.4.

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

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

injector/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.18.3 as builder
+FROM golang:1.21.4 as builder
 
 WORKDIR /build
 COPY go.mod go.sum ./