Remove symfony/http-client dependency

Marked as stable release after production deployments under PHP 7.4 and 8.1, and additional tests. This should not impose any additional dependencies, and will work with no changes to configuration. For Sagepay Server, your users will be redirected to a new https://live.opayo.eu.elavon.com/gateway/service/ URL, but the experience will be identical, even down to the Sagepay logos ;-)

Have fun.