Don't throw on ROOT::Experimental RNtuple warning

[dan131riley]

ROOT::Experimental::Detail::RPageSinkFile::RPageSinkFile(), in the RNtuple package

   R__WARNING_HERE("NTuple") << "The RNTuple file format will change. " <<
      "Do not store real data with this version of RNTuple!";

This should be added to the list of warnings that print instead of throwing.

Part of #103

[makortel]

With ROOT 6.34, I suppose we don't need to deal with this warning message anymore?

[dan131riley]

Since the storage format was frozen, this was removed in root-project/root#16769