net: use port 53 when checking for UDP routes

net: use port 53 when checking for UDP routes

Using port 9 is weird and at least once triggered a suspicious
activity alert.

Fixes #67264

Change-Id: If4179f054829c175b9f3a51c3bc2a3ca4afa74b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/584416
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>