The XMLRPC extension will be removed without deprecation in PHP 8.0.

This commit adds the functions and class from the extension to the respective RemovedFunctions and RemovedClasses sniffs.

The class is not listed in the PHP manual, but is in the stub file (as a final class), so, while I'm not 100% sure, I do think we should account for it.

Includes unit tests.

Refs:

• https://wiki.php.net/rfc/unbundle_xmlprc
• php/php-src@4fa3687
• https://www.php.net/manual/en/book.xmlrpc.php
• https://github.com/php/php-src/blob/444aa2636e81053e439623f2dbf981eaa48e6161/ext/xmlrpc/xmlrpc.stub.php

Related to #809