This change adds a PSR-15 middleware to intercept all /ter and /ter?wsdl
calls in favor of a pi-based plugin.

The plugin is removed (so the whole page in TYPO3 can be removed as well later-on).

The previous WSDL delivery endpoint extensions.typo3.org/typo3conf/ext/ter/tx_ter_wsdl.php
is now a redirect to /ter?wsdl, so everything is covered by the middleware.