Code Quality: Narrow $upgrader type in upgrader skins.
Each WP_Upgrader_Skin subclass is only ever constructed with one specific upgrader, but $upgrader was declared only on the parent as WP_Upgrader. Static analysis therefore flagged the subclass-specific calls to Plugin_Upgrader::plugin_info(), Theme_Upgrader::theme_info() and Language_Pack_Upgrader::get_name_for_update() as calls to undefined methods. Redeclaring the property in Language_Pack_Upgrader_Skin, Plugin_Upgrader_Skin, Plugin_Installer_Skin, Theme_Upgrader_Skin and Theme_Installer_Skin with its narrower type documents what was already true at runtime.
The @since tag on each redeclaration is the version in which that skin class was introduced rather than the current release, since the property has always been available by inheritance and is not newly added here.
With those five occurrences resolved, tests/phpstan/baselines/method.notFound.neon is empty and is removed along with its includes entry in phpstan.neon.dist, and two entries in property.notFound.neon that no longer match are dropped.
Developed in https://github.com/WordPress/wordpress-develop/pull/13283.
Follow-up to r63020.
Props nathanrice, westonruter.
See #65817.