Kubernetes Service as Sink only works with Port 80

**Describe the bug**
When using `v1/Service` and `kind: Service` as `sink`, i.e. Kubernetes Service instead of `KService`, all eventing components using that specific `sink` will only work if the `Service` is exposed on port `80` (`targetPort` can be different).

According to @n3wscott this is due to the specific Kubernetes `Service` resolver defaulting to port `80`, ignoring the specified port on that `Service` instance.

**Expected behavior**
Use defined port on Kubernetes `Service` instead of assuming `80`.

**To Reproduce**
Create a `Broker` and `Trigger` sending events to a `sink` defined as plain Kubernetes Service. The Kubernetes `Service` should be exposed on a port != `80`, e.g. `8080`. Again, `targetPort` (pod) does not matter.

**Knative release version**
Latest and master. (all?)

**Additional context**
From a discussion with @n3wscott

> refs resolve to port 80, you could use non-:80 if you use an abs url

It would be nice to get the correct behavior out of the box as I wasted an hour troubleshooting why a trigger would not deliver to my Kubernetes `Service` exposed on `8080` :) .

Start with the Kubernetes Service resolver mentioned in the report and trace how a plain v1/Service sink resolves its port. Reproduce the issue with a Service exposed on port 8080 and a Broker/Trigger, then verify that delivery uses the Service's defined port rather than assuming port 80.

Written by the indexing model from the issue text.