xF2 Add-on AndyB Version Check 2.0

Version check v2.0 changes:

Ensure only xf2addon.com are shown.

Version check v1.9 changes:

Now uses XenForo temporary directory and file functions.

Version check v1.8 changes:

Fixed issue where PHP 8 can throw a server error under certain conditions.

Version check v.17 changes:

Removed dependency to use file_get_contents.