chore(deps): update alpine docker tag to v3.21 (#14995)

This PR contains the following updates:

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

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crates/cargo-test-support/containers/sshd/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20
+FROM alpine:3.21
 
 RUN apk add --no-cache openssh git
 RUN ssh-keygen -A