[release-branch.go1.16] std: update golang.org/x/net to 20210428183300-3f4a416c7d3b

Steps:
  go get -d golang.org/x/net@internal-branch.go1.16-vendor
  go mod tidy
  go mod vendor

This http2 bundle does not need to be updated.

Fixes #45712

Change-Id: I7c86c31248e0cd250d17495c3f985671cfcf44a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/314789


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>