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

[Soean]

is_integer() is an alias for 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.

There is also a proposal to deprecate is_integer() in PHP 8.6, see RFC

I am proposing to replace all occurrences of is_integer() with is_int() throughout the WordPress Core. This is a non-breaking change as the functionality remains exactly the same.

Trac ticket: https://core.trac.wordpress.org/ticket/64913

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