DPL: more fixes to handle the ROOT v6-36-00

[ktf]

• More tweaks to get ROOT::Experimental out of the way
• RNTuples cannot have / as first character in name

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[ktf]

@pzhristov this should solve the remaining issues on my side for the new ROOT.

[pzhristov]

@ktf It seems we still have the problem in framework:root. Do we need PR #14332 for the successful completion of the tests?

[ktf]

I fear some of my changes for the new ROOT broke the old one. Checking.

[ktf]

#14332 is unrelated. Originally, it happened to crash in the same place, which is why I was originally confused, and it should now be good to go.

[pzhristov]

@ktf I confirm that this PR fixes the issue of framework:root with Root v6-36-00.

[ktf]

Code not used / unchanged until we switch to ROOT v6-36-00.

[ktf]

Also tested on hyperloop.