fix(CLI): correct gateway api version in user instructions

[adleong]

When a user runs linkerd install --crds but does not have the Gateway API CRDs installed on the cluster, Linkerd prints this error message:

The Gateway API CRDs must be installed prior to installing Linkerd. Run:

kubectl apply --server-side -f https://github.com/kubernetes-sigs/gateway-api/releases/download/v1.4.0/standard-install.yaml

or see https://gateway-api.sigs.k8s.io/guides/#installing-gateway-api for more options.

However, the Gateway API version v1.4.0 is outside of the range of versions that Linkerd supports.

Replace this version with the maximum supported Gateway API version of v1.2.1.