Sergey Biryukov ab263f93e5 Upgrade/Install: Check if the disk_free_space() function exists before calling it.
In PHP 8+, `@` no longer suppresses fatal errors:
> The `@` operator will no longer silence fatal errors (`E_ERROR`, `E_CORE_ERROR`, `E_COMPILE_ERROR`, `E_USER_ERROR`, `E_RECOVERABLE_ERROR`, `E_PARSE`).

Reference: [https://www.php.net/manual/en/migration80.incompatible.php PHP 8: Backward Incompatible Changes].

`disk_free_space()` may be disabled by hosts, which will throw a fatal error on a call to undefined function.

This change prevents the fatal error, and falls back to `false` when `disk_free_space()` is unavailable.

Follow-up to [25540], [25774], [25776], [25831], [25869].

Props costdev, jrf, swb1192, SergeyBiryukov.
Fixes #54826. See #54730.
Built from https://develop.svn.wordpress.org/trunk@52585


git-svn-id: http://core.svn.wordpress.org/trunk@52175 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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