net/http: update bundled x/net/http2

This updates x/net/http2 to x/net git rev 351d144f for:

   http2: revert Transport's strict interpretation of MAX_CONCURRENT_STREAMS
   https://golang.org/cl/151857

   http2: don't leak streams on broken body
   https://golang.org/cl/132715

   http2: remove support for Go 1.8 and earlier
   https://golang.org/cl/145677

   http2: reduce init-time work & allocations
   https://golang.org/cl/127664

And some CLs fixing typos.

Fixes #27044
Fixes #27208

Change-Id: I11cc32576c690199ceb4c0bd1448d01e3cab3097
Reviewed-on: https://go-review.googlesource.com/c/152080
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

Author: bradfitz