Code Quality: Replace `is_integer()` with `is_int()` for consistency.

[https://www.php.net/manual/en/function.is-integer.php is_integer()] is an alias for [https://www.php.net/manual/en/function.is-int.php is_int()]. While they function identically, the WordPress Coding Standards and modern PHP practices generally lean towards using the official function name rather than its alias to maintain consistency across the codebase.

Props Soean, westonruter, SergeyBiryukov.
See #64913.

git-svn-id: https://develop.svn.wordpress.org/trunk@62175 602fd350-edb4-49c9-b593-d223f7449a82