[release-branch.go1.15] net/http: update bundled x/net/http2

Bring in the change in CL 304309 with:

	go mod edit -replace=golang.org/x/net=golang.org/x/net@release-branch.go1.15-bundle
	GOFLAGS='-mod=mod' go generate -run=bundle std
	go mod edit -dropreplace=golang.org/x/net
	go get -d golang.org/x/net@release-branch.go1.15
	go mod tidy
	go mod vendor

For #45076.
Updates #40213.

Change-Id: I68d5e1f2394508c9cf8627fb852dd9e906d45016
Reviewed-on: https://go-review.googlesource.com/c/go/+/305489


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