Code Modernization: Use explicit visibility for class property declarations.

Using `var` or only `static` to declare a class property is PHP 4 code.

This updates the codebase to use explicit visibility modifiers introduced in PHP 5.

Props jrf.
Fixes #51557. See #22234.

git-svn-id: https://develop.svn.wordpress.org/trunk@49184 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/factory/class-wp-unittest-generator-sequence.php

@@ -1,7 +1,7 @@
 <?php
 
 class WP_UnitTest_Generator_Sequence {
-	static $incr = -1;
+	public static $incr = -1;
 	public $next;
 	public $template_string;