Replace deprecated `prune-whitelist` flag with `prune-allowlist` for `kubectl` command

[roger2hk]

Warning message from the Google Cloud Build:

Step #10 - "apply_k8s_cfgs_for_mysql_dryrun": Running: kubectl apply --dry-run=server --namespace=mysql -f=envsubst-mysql/etcd-cluster.yaml -f=envsubst-mysql/trillian-ci-mysql.yaml -f=envsubst-mysql/trillian-mysql.yaml -f=envsubst-mysql/trillian-log-deployment.yaml -f=envsubst-mysql/trillian-log-service.yaml -f=envsubst-mysql/trillian-log-signer-deployment.yaml -f=envsubst-mysql/trillian-log-signer-service.yaml --prune --all --prune-whitelist=core/v1/ConfigMap
Step #10 - "apply_k8s_cfgs_for_mysql_dryrun": Flag --prune-whitelist has been deprecated, Use --prune-allowlist instead.

Checklist

• [ ] I have updated the CHANGELOG.
• [ ] I have updated documentation accordingly (including the feature implementation matrix).

[roger2hk]

trillian-pr-deploy (trillian-opensource-ci) is failed due to the deprecated GKE version running behind the scene.

Step #10 - "apply_k8s_cfgs_for_mysql_dryrun": Running: kubectl apply --dry-run=server --namespace=mysql -f=envsubst-mysql/etcd-cluster.yaml -f=envsubst-mysql/trillian-ci-mysql.yaml -f=envsubst-mysql/trillian-mysql.yaml -f=envsubst-mysql/trillian-log-deployment.yaml -f=envsubst-mysql/trillian-log-service.yaml -f=envsubst-mysql/trillian-log-signer-deployment.yaml -f=envsubst-mysql/trillian-log-signer-service.yaml --prune --all --prune-allowlist=core/v1/ConfigMap
Step #10 - "apply_k8s_cfgs_for_mysql_dryrun": error: unknown flag: --prune-allowlist

https://github.com/kubernetes/kubernetes/pull/113116

[roger2hk]

The GKE version is now 1.26 in GCP, so the check is now passed.