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.

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/main.cpp

@@ -104,7 +104,7 @@ int main(int argc, char** argv)
         builder.append(namespace_);
         builder.append("::"sv);
         builder.append(interface.name);
-        interface.fully_qualified_name = builder.to_string();
+        interface.fully_qualified_name = builder.to_deprecated_string();
     } else {
         interface.fully_qualified_name = interface.name;
     }