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/Impl/WebSocketImplSerenity.cpp

@@ -83,7 +83,7 @@ ErrorOr<DeprecatedString> WebSocketImplSerenity::read_line(size_t size)
 {
     auto buffer = TRY(ByteBuffer::create_uninitialized(size));
     auto line = TRY(m_socket->read_line(buffer));
-    return line.to_string();
+    return line.to_deprecated_string();
 }
 
 }