[release-branch.go1.15] std: update golang.org/x/net to 20210428183841-261fb518b1ed

Steps:
  go get -d golang.org/x/net@release-branch.go1.15
  go mod tidy
  go mod vendor

This http2 bundle does not need to be updated.

Fixes #45711

Change-Id: I085ca592dfc8d5d9c328a7979142e88e7130a813
Reviewed-on: https://go-review.googlesource.com/c/go/+/314790


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