proxy: User ReverseProxy.Rewrite instead of Director

With Go 1.20 the "Rewrite" hook for ReverseProxy was introduced to
supersede of the "Director" hook (see:
https://github.com/golang/go/commit/a55793835f16d0242be18aff4ec0bd13494175bd)

The Rewrite hooks allows for better separation between the incoming and
outgoing request. In particular it makes it pretty easy to set the
correct X-Forwarded-* Headers on the outgoing request.
The need for using "Rewrite" came up when trying to embed
authelia. It uses the X-Forwarded-Host and X-Forwared-Proto headers to
e.g. compute the correct return values for the various endpoints in
.well-known/openid-configuration.

services/proxy/pkg/router/router_test.go

@@ -1,9 +1,11 @@
 package router
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"net/http/httputil"
 	"net/url"
 	"testing"
 
@@ -146,10 +148,14 @@ func TestRouter(t *testing.T) {
 			t.Errorf("TestRouter route flag unprotected expected to be %t got %t", test.unprotected, routingInfo.IsRouteUnprotected())
 		}
 
-		routingInfo.Director()(r)
+		pr := &httputil.ProxyRequest{
+			In:  r,
+			Out: r.Clone(context.Background()),
+		}
+		routingInfo.Rewrite()(pr)
 
-		if r.URL.Host != test.target {
-			t.Errorf("TestRouter got host %s expected %s", r.URL.Host, test.target)
+		if pr.Out.URL.Host != test.target {
+			t.Errorf("TestRouter got host %s expected %s", pr.Out.URL.Host, test.target)
 		}
 	}
 }