net/http: parse HTTP version strings according to RFC 7230

net/http: parse HTTP version strings according to RFC 7230

RFC 2616 permits multiple digits in the major and minor numbers of an
HTTP version:

	https://datatracker.ietf.org/doc/html/rfc2616#section-3.1

RFC 7230 obsoletes 2616 and tightens the specification to permit only a
single digit in the major and minor number:

	https://datatracker.ietf.org/doc/html/rfc7230#section-2.6

Use the stricter definition.

Also fix a bug which caused version numbers with a leading "+" to
be accepted (e.g., "HTTP/1.+1".)

Fixes #46587.

Change-Id: Ic5923bb858e5ac402cfde486fba2c075e221553d
Reviewed-on: https://go-review.googlesource.com/c/go/+/325874
Trust: Damien Neil <dneil@google.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>