internal/redirect: bump bigEnoughAssumeRietveld

Gerrit CL numbers now exceed 300k, so new CL numbers are landing on the
Gerrit/Rietveld disambiguation page.

Bump bigEnoughAssumeRietveld to 4M (approximately the max before we need
to handle subrepos) and update the table. Now we won't need to deal with
this again for quite a while.

Change-Id: I6ddd8cdd19012f0b58e39c2df76ff23f376c2f1b
Reviewed-on: https://go-review.googlesource.com/c/website/+/300089
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

internal/redirect/redirect_test.go

@@ -69,8 +69,8 @@ func TestRedirects(t *testing.T) {
 		"/cl/152048": {302, "https://codereview.appspot.com/152048"},
 
 		// And verify we're using the "bigEnoughAssumeRietveld" value:
-		"/cl/299999": {302, "https://go-review.googlesource.com/299999"},
-		"/cl/300000": {302, "https://codereview.appspot.com/300000"},
+		"/cl/3999999": {302, "https://go-review.googlesource.com/3999999"},
+		"/cl/4000000": {302, "https://codereview.appspot.com/4000000"},
 	}
 
 	mux := http.NewServeMux()