build(deps): bump alpine from 3.20 to 3.21 in /cfssl

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]>

cfssl/Dockerfile

@@ -17,7 +17,7 @@ RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags "-s -w
 RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags "-s -w -X github.com/cloudflare/cfssl/cli/version.version=1.6.5" -o /out/mkbundle ./cmd/mkbundle
 RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -ldflags "-s -w -X github.com/cloudflare/cfssl/cli/version.version=1.6.5" -o /out/multirootca ./cmd/multirootca
 
-FROM alpine:3.20 AS cfssl-server
+FROM alpine:3.21 AS cfssl-server
 
 COPY --from=builder /out/* /
 
@@ -27,7 +27,7 @@ EXPOSE 8080
 
 CMD ["/cfssl"]
 
-FROM alpine:3.20 AS cfssl
+FROM alpine:3.21 AS cfssl
 
 COPY --from=builder /out/* /