Replace github.com/gorilla/websocket in Workhorse
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The [Gorilla Web Toolkit](https://github.com/gorilla#gorilla-toolkit) has been put in read-only mode. The code is still available and still works, but there are no maintainers anymore and it will not receive updates.
Workhorse uses Gorilla to handle websocket connections. It would be good if we could modify this code to no longer use Gorilla.
Workhorse also has indirect dependencies on Gorilla at the module level but those do not get compiled into any of our binaries so we do not have to worry about them.
1. Pick a replacement library
2. Convert top-level integration tests to use new library
3. Convert `internal/upstream` to use new library
4. Convert `internal/channel` to use new library
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