Modernize: use numeric literal separators

Since PHP 7.4, PHP allows using numeric literal separators in integers and floats to improve the readability of long numbers.

As this codebase has dropped support for PHP < 7.4 by now, we can apply this modernization to all long numbers in this codebase.

Ref:
* https://wiki.php.net/rfc/numeric_literal_separator

Author: jrfnl

tests/WP/Handlers/Save_Post_Handler_Test.php

@@ -228,7 +228,7 @@ public function test_delete_on_save_post_does_nothing_for_nonexistent_post() {
 		$_POST['duplicate_post_remove_original'] = '1';
 
 		// Call the method with a non-existent post ID - should not throw error.
-		$this->instance->delete_on_save_post( 999999 );
+		$this->instance->delete_on_save_post( 999_999. );
 
 		// If we get here without errors, the test passes.
 	}