After [42832], [42891] and [43006], this changeset refines the core notice informing about an outdated PHP version:
* The link to the Update PHP information page can now be adjusted using either a `WP_UPDATE_PHP_URL` environment variable, or a new `wp_update_php_url` filter.
* If that URL is different from the default one that points to https://wordpress.org/support/update-php/ or its localized equivalent, a note indicates that the linked resource has not been provided by WordPress itself, and the default URL is still linked to as an additional resource.
* The URL for the default information page has been updated to use the slug `update-php` instead of `upgrade-php`.
* `@since` annotations have been updated.
Going forward, admin areas that display information related to the PHP version should use the new function `wp_get_update_php_url()`.
Props afragen, fierevere, flixos90, markjaquith, miss_jwo, nerrad, pento, schlessera, SergeyBiryukov, spacedmonkey.
Fixes#45686. See #41191.
git-svn-id: https://develop.svn.wordpress.org/trunk@44476 602fd350-edb4-49c9-b593-d223f7449a82
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