net/http: update bundled http2

Update bundled x/net/http2 to x/net git rev 6c4ac8bd for:

   http2: fix Transport race sending RST_STREAM while reading DATA on cancels
   https://golang.org/cl/32571

   http2: remove h2-14 ALPN proto
   https://golang.org/cl/32576

Fixes #16974

Change-Id: I6ff8493a13d2641499fedf33e8005004735352ff
Reviewed-on: https://go-review.googlesource.com/32578


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>