docs(consul-api-gateway): add v0.4.0 upgrade instructions

[mikemorris]

Description

Adding instructions for upgrading to Consul API Gateway v0.4.0

Testing & Reproduction steps

Wrote the instructions by taking notes as I upgraded Consul API Gateway to a newer binary, then upgraded resources, on a local kind cluster.

Abandoned Ideas

• kube-storage-version-migrator Successfully updates the GatewayClass, Gateway and HTTPRoute resources in a cluster from v1alpha2 to v1beta1 when the new Gateway API CRDs are installed, but is fully automated and has a cluster-wide scope - it attempts to automatically update every resource in the cluster, which would likely be an unexpected UX for an operator intending to update just a few specific resources.
• kubectl-convert Doesn't support updating CRDs, see https://github.com/kubernetes/kubernetes/issues/55801

PR Checklist

• [ ] ~~updated test coverage~~
• [x] external facing docs updated
• [x] not a security concern