net/http: mark http/2 connections active

On Server.Shutdown, all idle connections are closed.
A caveat for new connections is that they are marked idle
after 5 seconds.
Previously new HTTP/2 connections were marked New, and after 5 seconds,
they would then become idle. With this change, we now mark HTTP/2
connections as Active to allow the proper shutdown sequence to occur.

Fixes #36946
Fixes #39776

Change-Id: I31efbf64b9a2850ca544da797f86d7e1b3378e8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/240278


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>