net/http: return 505 status for rejected protocol version

When rejecting a request with an unsupported HTTP protocol version,
return a 505 error ("HTTP Version Not Supported") instead of 400.

Fixes #40454.

Change-Id: I0269f0f5755d90d1b772ba0094a6bb24b5eb4701
Reviewed-on: https://go-review.googlesource.com/c/go/+/261977


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Urban Ishimwe <urbainishimwe@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>