Rename `upgrade_php` capability to `update_php`

[flixos90]

In today's PHP meeting, we decided to use the term "update" instead of "upgrade" throughout. This is in line with how WordPress has commonly used only "update", with "upgrade" only appearing sometimes internally in the code.

Similar capabilities are called update_core, update_plugins etc., so our capability should be in line with this too. While we initially considered "upgrade" a more fitting term for PHP, eventually consistency makes sense here, and users are more familiar already with the term "update".

This is a follow-up to #41191. Since the capability has not been part of any WordPress release and has not been used outside of its single use-case, we can easily rename it without backward-compatibility implications.

[flixos90]

44457.diff​ changes the name, and also includes a CS violation fix in affected code.

[flixos90]

In 43381:

Role/Capability: Rename upgrade_php capability to more suitable update_php.

This brings the name in line with user-facing language and similar names of existing related capabilities. Since the capability has not been part of any WordPress release, it can be renamed without any backward-compatibility implications.

Also missing props benhuberman for [43006].

Fixes #44457.

[flixos90]

Via reopening of #41191, this will be part of 5.1.