Make the update-php template part of wporg, instead of wporg-support

[Clorith]

in r8071 (for #4004) the update-php page was implemented as a page template for use with the wporg-support theme.

Unfortunately, not all locales use support forums, either because they don't see a need, or they have their own community support solution in place. This limits them from providing the new page template in their own language for WordPress 5.1 and onwards when the update notice goes live.

Instead we should look at moving it away from the helphub/support theme, and to the main wporg theme that rosetta sites have access to, so it's available to all locales.

[fierevere]

related: https://meta.trac.wordpress.org/ticket/4139

[Clorith]

Duplicate of #4139.