[release-branch.go1.16] net/http: update bundled golang.org/x/net/http2

Pull in approved backports to golang.org/x/net/http2:

    64539c1 http2: don't count aborted streams as active in tests
    e677a40 ipv6: OpenBSD does not appear to support multicast loopback
    d8ae719 net/http2: Fix handling of expect continue
    cc2f99c http2: avoid busy loop when ResponseHeaderTimeout is set
    5533dda http2: avoid spurious RoundTrip error when server closes and resets stream
    26ec667 http2: close conns after use when req.Close is set

By doing:

    $ go get -d golang.org/x/net@internal-branch.go1.16-vendor
    go: downloading golang.org/x/net v0.0.0-20211201233224-64539c132272
    go get: upgraded golang.org/x/net v0.0.0-20211101194150-d8c3cde3c676 => v0.0.0-20211201233224-64539c132272
    $ go mod tidy
    $ go mod vendor
    $ go generate -run=bundle std

Fixes #49904.
Fixes #49623.
Fixes #49661.
Fixes #49560.
Fixes #49908.
Fixes #49910.

Change-Id: I73261b189f84cf1919a79129ec36a1c187723133
Reviewed-on: https://go-review.googlesource.com/c/go/+/368594


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>