Fix a data race in GrpcConnection when retrieving the language token. (#8257)

This is a backport of cr/377421930.

#no-changelog

Author: var-const

Firestore/core/src/remote/grpc_connection.cc

@@ -189,7 +189,7 @@ class ClientLanguageToken {
     value_ = std::move(value);
   }
 
-  const std::string& Get() const {
+  std::string Get() const {
     Guard guard(mutex_);
     return value_;
   }