Undo the gorouter changes for 100-continue that are no longer needed from go 1.22.5

[geofffranks]

Proposed Change

Context

In #418, #419, #420, and #424, we worked around an issue in Golang by modifying the reverseproxy logic to make use of Director (for most cases) + Redirector (for 100-continue cases). Now that Golang has fixed the underlying issue, we can back out this workaround.

What

Revert all gorouter code changes (but not tests) from #418, #419, #420 and #424. Ensure that the tests added in #418, #419, #420, and #424 remain + pass.

Acceptance criteria

Tests should still pass

Related links

No response

[geofffranks]

FYI beginning work on this

[geofffranks]

Ok, the code is mostly done on https://github.com/cloudfoundry/gorouter/tree/revert-proxy-rewrite-divergence. However, our integration test is causing a race condition in Golang's http.Server to be triggered when Flushing buffers. We believe this will be fixed in golang 1.23 as part of other mutexes/locking of the http request/response buffers already done.

Pausing this until gorouter is updated to 1.23, and then resuming after that to make sure everthing is happy again.