Use port number instead of name for k8s service port (#3256)

I've found an issue when deploying Dendrite's Helm chart on my local
cluster. The template for generating an Ingress resource tries to find
the service port using a name (`http`), but the template that generates
the Service resource, instead, identifies the resource with a port
number.

According to the [Kubernetes
ServiceSpec](https://kubernetes.io/docs/reference/kubernetes-api/service-resources/service-v1/),
`ports.targetPort` can be either a number or a string; if it's the
latter, it will be looked up as a named port in the pod's container
ports.

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[skip ci]

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