The [ReverseProxy.Director] configuration field is deprecated
in favor of [ReverseProxy.Rewrite].

A malicious client can remove headers added by a `Director` function
by designating those headers as hop-by-hop. Since there is no way to address
this problem within the scope of the `Director` API, we added a new
`Rewrite` hook in Go 1.20. `Rewrite` hooks are provided with both the
unmodified inbound request received by the proxy and the outbound request
which will be sent by the proxy.

Since the `Director` hook is fundamentally unsafe, we are now deprecating it.