Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibWebSocket/WebSocket.cpp

@@ -213,7 +213,7 @@ void WebSocket::send_client_handshake()
     builder.append("\r\n"sv);
 
     m_state = WebSocket::InternalState::WaitingForServerHandshake;
-    auto success = m_impl->send(builder.to_string().bytes());
+    auto success = m_impl->send(builder.to_deprecated_string().bytes());
     VERIFY(success);
 }