Make sure RNTupleReader build with ROOT > 6.34

[tmadlener]

BEGINRELEASENOTES

• Make sure that the RNTupleReader still builds with ROOT > 6.34

ENDRELEASENOTES

RException (and other RNTuple API parts) have been moved out of the Experimental namespace with root-project/root#17281

[tmadlener]

It looks like some ov the dev3 stacks have not yet caught up with the latest ROOT (i.e. the latest symlink still points to beginning of the week, e.g. Mon or Tue for some of the stacks. x86_64-ubuntu2404-gcc13-opt is already on Thu and this works there).

@andresailer just in case you are not already aware of this.

[jmcarcell]

Since there are going to be several changes like this one before the next version maybe it's simpler to keep a single PR instead of making N PRs.

[tmadlener]

Yeah probably a good idea. Depending on how spread out the changes are, we will probably have to split it into a few. Keeping this open to collect a few of the things for now.