configitems: enable changegroup3 by default (unless using infinitepush) The LFS extension requires this, and if it isn't enabled on the client (or the LFS extension isn't loaded), a web client gets a 500 instead of a sensible error message. Now it gets a different (client) error, but maybe it can be handled more gracefully. c0f11347b107 indicates that treemanifest repos have this issue too. 29cfc474c5fd mentions gating this behind `experimental` so that the format could change, but that was 7 years ago and we now have an experimental `changegroup4` as well. We can keep this as a config for the next cycle in case someone runs into an unexpected problem, and then jettison it if the infinitepush bundle name changes are either acceptable as-is or can be created differently.