chore(deps): update alpine docker tag to v3.21 (kubectl-helm-az/dockerfile)

Author: IK Renovate Bot

kubectl-helm-az/Dockerfile

@@ -4,7 +4,7 @@ FROM lachlanevenson/k8s-kubectl:v1.25.4
 # We build our own base az alpine image becase all official images currently
 # have security vulnerabilities (as of mcr.microsoft.com/azure-cli:2.0.64).
 # Ref: https://github.com/Azure/azure-cli/issues/9167
-FROM alpine:3.20
+FROM alpine:3.21
 ENV AZ 2.0.64
 RUN apk add -U python3 bash ca-certificates && \
   apk add --virtual=build gcc python3-dev musl-dev libffi-dev openssl-dev make  && \