Bump alpine from 3.20 to 3.21

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]>

Dockerfile

@@ -11,7 +11,7 @@ COPY go.sum go.sum
 RUN go get -d -v
 RUN CGO_ENABLED=0 GOOS=linux go build --ldflags '-w -s -extldflags "-static"' -o cloudflare_exporter .
 
-FROM alpine:3.20
+FROM alpine:3.21
 
 RUN apk update && apk add ca-certificates