Rename Update PHP slug and redirect previous one

[flixos90]

As discussed before, the URL slug of the Update PHP page (see ​https://wordpress.org/support/upgrade-php/) should no longer be upgrade-php, but instead update-php. A redirect from the old URL to the new one needs to be set up on .org, and the page slug has to be changed.

This is a follow up to #2996

[SergeyBiryukov]

In 7987:

Support Hub: Redirect "Update PHP" page from the old slug to the new one.

The old slug is upgrade-php, the new one is update-php.

Fixes #4005.

[SergeyBiryukov]

In 7988:

Support Hub: Avoid redirecting the old slug of "Update PHP" page to a forum topic.

See #4005.