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

Pull in security fix

    84cba54 http2: cap the size of the server's canonical header cache

Updates #50058
Fixes CVE-2021-44716

Change-Id: Ia89e3d22a173c6cb83f03608d5186fcd08f2956c
Reviewed-on: https://go-review.googlesource.com/c/go/+/370574


Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>