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

Updates bundled http2 to x/net git rev 16c2bbf55 for:

	http2: send a nil error if we cancel a delayed body write
	https://golang.org/cl/288013

	http2: wait until the request body has been written
	https://golang.org/cl/288012

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

Fixes golang/go#42539

Change-Id: I299c6d4a67ebc036e45c978e4d03cba73717b363
Reviewed-on: https://go-review.googlesource.com/c/go/+/288112


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>