* chore(helm)!: change iptables default mode to `nft`
This change sets as a new default `nft` for the `proxyInit.iptablesMode`
and `iptablesMode` values in the linkerd-control-plane and linkerd2-cni
helm charts. This doesn't imply any change in user-facing behavior.
This was prompted by EKS with k8s 1.33 no longer supporting the iptables
legacy mode.
Further testing in multiple platforms with different k8s versions
revealed nft mode is now broadly supported.
Upgrading via Helm will apply the new default, unless the initial
install explicitly set the legacy mode.
