net/http: update bundled x/net/http2

This updates the bundled copy of x/net/http2 to x/net git rev a8e8f92cd6 for:

    http2: remove extra goroutine stack from awaitGracefulShutdown
    https://golang.org/cl/43230

    http2: Discard DATA frames from the server after the response body is closed
    https://golang.org/cl/43810

Fixes #20302
Fixes #18471
Fixes #20448

Change-Id: I00972836deb2fe6049f631ee44901732a641b171
Reviewed-on: https://go-review.googlesource.com/44006Reviewed-by: Tom Bergan <tombergan@google.com>