Websockets do not work with domainmapings by default

## What version of Knative?

> 1.5

## Expected Behavior

Websockets to work out of the box when using domain mappings. Especially since websockets work with the default endpoints that are created.

## Actual Behavior

Websockets ONLY work with the default auto generated endpoint, and return a 503 when using a domain mapping.

## Steps to Reproduce the Problem

This is very similar to https://github.com/knative/serving/issues/7933, however that focuses on GKE. The problem seems to exist for all installations. In my case, I am using RKE2 on vultr.

The exact service config I am deploying is here: https://gist.github.com/georgyo/846e72c94ed20b4d2988a2f164f31c4b/64742e75d9ea78b63a3af261784beb6e48e2ccaa

This creates two endpoints
- https://wrtc-star.default.k.fu.io (works)
- https://wrtc-star.scalable.io (does not work)

They are pointing to the same service, and as such you would expect both to work or neither to work.

Start by reproducing the issue with the service configuration in the linked gist, comparing the working default endpoint with the domain-mapped endpoint and its 503 response. Review the related serving issue 7933 for context; done means WebSocket connections work through both endpoints, not only the autogenerated one.

Written by the indexing model from the issue text.