http2: enable net/http wrapping when go >= 1.27

When building with Go 1.27 or newer, use an HTTP/2
client and server which are implemented in terms of net/http.

The original source of truth for the Go HTTP/2 implementation
was the x/net/http2 package. A copy of this package was included
in the standard library.

In Go 1.27, the source of truth of the HTTP/2 implementation
has moved entirely into the standard library. There are no
supported features of the HTTP/2 implementation which require
importing x/net/http2, and most users should just use net/http
directly.

We still support existing code which imports x/net/http2.
To avoid maintaining two divergent HTTP/2 implementations,
x/net/http2 (when used with a recent enough version of Go)
now just wraps net/http.

For now, users may revert to the the pre-Go 1.27 implementation
by setting the build tag "http2legacy":

  go build -tags=http2legacy ...

For golang/go#78508

Change-Id: I6efeddc4f3bcba9388c19cbcd1e9b8a56a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/774440
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>