Fix PHP 8.2 deprecation warning

[DanieleAlessandra]

The report on PHP 8.2 deprecation warnings for dynamic properties highlighted three cases, which this PR addresses:

1. FS_Plugin_Plan::pricing – This was resolved in June.
2. FS_Site::blog_id – This issue was due to direct assignment.
3. FS_User::is_beta – This resulted from the use of unserialize.

Changes:

• Removed the @property FS_Pricing[] $pricing annotation from FS_Plugin_Plan, as the property now exists and the annotation is no longer necessary.
• Added the $blog_id property to FS_Site and removed the corresponding annotation, similar to the previous class.
• Introduced the __wakeup magic method to FS_User to remove deprecated properties from serialized entities.
• Added the __unserialize magic method to FS_User to manage the result of unserialize and prevent warnings. This method only works with public properties and can be removed if the __wakeup method is sufficient. __unserialize magic method is detected as non-magic in PHP 7.3 and older by phpcs.