[repository,tests] use the new connection api (part 1/2) `add_info` should not play with the cnxset nor the mode. Static analysis showed that: * add_info is an internal api called with a non-none cnx.cnxset and also in write mode * only the altered test violated this assumption, which is now somewhat documented

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raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")