Add support for HTTP/2 to status_header()

[johnbillion]

The status_header() function only outputs one of HTTP/1.0 {status} or HTTP/1.1 {status} based on the requested protocol in $_SERVER['SERVER_PROTOCOL'].

This function should also support HTTP/2 and HTTP/2.0 (Nginx trunk is currently using HTTP/2.0 which ​I'm not sure is correct).

In addition, $_SERVER['SERVER_PROTOCOL'] is used in three other places in core, so these should be updated too.

[wonderboymusic]

any benefit to adding a wrapper func?

[wonderboymusic]

34131.2.diff​ DRYs it.

[wonderboymusic]

In 34894:

Introduce wp_get_server_protocol() to DRY protocol parsing logic and make adding more protocols, like HTTP/2, easier.

Props johnbillion, wonderboymusic.
Fixes #34131.