Replace SimpleSAML_Session with namespaced version

docs/simplesamlphp-advancedfeatures.md

docs/simplesamlphp-maintenance.md

docs/simplesamlphp-sp.md

lib/SimpleSAML/Auth/Default.php

lib/SimpleSAML/Auth/Simple.php

lib/SimpleSAML/Auth/Source.php

lib/SimpleSAML/Auth/State.php

lib/SimpleSAML/Error/Error.php

lib/SimpleSAML/IdP.php

lib/SimpleSAML/Logger.php

lib/SimpleSAML/Session.php

lib/SimpleSAML/SessionHandler.php

lib/SimpleSAML/SessionHandlerCookie.php

lib/SimpleSAML/SessionHandlerPHP.php

lib/SimpleSAML/SessionHandlerStore.php

lib/SimpleSAML/Utilities.php

lib/SimpleSAML/Utils/Auth.php

lib/SimpleSAML/Utils/HTTP.php

lib/SimpleSAML/XHTML/IdPDisco.php

modules/core/lib/Auth/Process/ExtendIdPSession.php