[release-branch.go1.17] net/http: update bundled golang.org/x/net/http2

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

    21a9c9c http2: prioritize RST_STREAM frames in random write scheduler

By doing:

    $ go get -d golang.org/x/net@internal-branch.go1.17-vendor
    $ go mod tidy
    $ go mod vendor
    $ go generate -run=bundle std

Fixes #49921

Change-Id: I04739a30d84a8ae449374eca8bb11c7d2d215ad9
Reviewed-on: https://go-review.googlesource.com/c/go/+/375814


Run-TryBot: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Carlos Amedee <carlos@golang.org>