chore(deps): update alpine docker tag to v3.21 (open-telemetry#36741)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| alpine | final | minor | `3.20` -> `3.21` |

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examples/demo/client/Dockerfile

@@ -5,6 +5,6 @@ WORKDIR /app/
 COPY . .
 RUN go env -w GOPROXY=direct
 RUN CGO_ENABLED=0 go build -o main main.go
-FROM alpine:3.20
+FROM alpine:3.21
 COPY --from=build /app/main  /app/main
 CMD ["/app/main"]

examples/demo/server/Dockerfile

@@ -5,6 +5,6 @@ WORKDIR /app/
 COPY . .
 RUN go env -w GOPROXY=direct
 RUN CGO_ENABLED=0 go build -o main main.go
-FROM alpine:3.20
+FROM alpine:3.21
 COPY --from=build /app/main  /app/main
 CMD ["/app/main"]