[release-branch.go1.14] net/http: update bundled x/net/http2

Bring in the change in CL 266157 with:

	go get -d golang.org/x/net@release-branch.go1.14
	go mod tidy
	go mod vendor
	go generate -run=bundle std

Updates #39337.
Fixes #42112.

Change-Id: Iefd0012369c7f0c58201256e29d21210cb9f2f7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/266374


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>