[release-branch.go1.15] src, net/http: update vendor, regenerate h2_bundle.go

Features CL:

    net/http2: send WINDOW_UPDATE on a body's write failure
    https://golang.org/cl/258478 (updates #41387)

Created by:

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

Updates #40423
Fixes #41387

Change-Id: I052037d6b6ed38b9d9782e19b8ce283875354c92
Reviewed-on: https://go-review.googlesource.com/c/go/+/258540


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