Bump alpine from 3.20 to 3.21 in /cmd/otel-allocator (#3526)

Bumps alpine from 3.20 to 3.21.

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

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

Author: dependabot[bot]

cmd/otel-allocator/Dockerfile

@@ -1,5 +1,5 @@
 # Get CA certificates from the Alpine package repo
-FROM alpine:3.20 AS certificates
+FROM alpine:3.21 AS certificates
 
 RUN apk --no-cache add ca-certificates