Code Modernization: Explicitly declare WP-specific property in WP_SimplePie_File.

Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.

There are a number of ways to mitigate this:
* If it is an accidental typo for a declared property: fix the typo.
* For known properties: declare them on the class.
* For unknown properties: add the magic `__get()`, `__set()`, et al. methods to the class or let the class extend `stdClass` which has highly optimized versions of these magic methods built in.
* For unknown use of dynamic properties, the `#[AllowDynamicProperties]` attribute can be added to the class. The attribute will automatically be inherited by child classes.

In this case, the `$timeout` property falls in the “known property” category.

Note: In contrast to the other properties being set in the constructor, this property is not declared on the parent class, so not inherited.

Reference: [https://wiki.php.net/rfc/deprecate_dynamic_properties PHP RFC: Deprecate dynamic properties].

Follow-up to [10687], [53557], [53558], [53850], [53851], [53852], [53853], [53854], [53856], [53916], [53935], [53936], [53937], [53938], [53942], [53945], [53948], [53949], [53952].

Props jrf, antonvlasenko, costdev.
See #56033.

git-svn-id: https://develop.svn.wordpress.org/trunk@53953 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-simplepie-file.php

@@ -19,6 +19,13 @@
  */
 class WP_SimplePie_File extends SimplePie_File {
 
+	/**
+	 * Timeout.
+	 *
+	 * @var int How long the connection should stay open in seconds.
+	 */
+	public $timeout = 10;
+
 	/**
 	 * Constructor.
 	 *