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.

--- a/tests/test-ssh.t	Mon Feb 27 19:59:55 2023 -0500
+++ b/tests/test-ssh.t	Mon Feb 27 23:04:09 2023 -0500
@@ -529,7 +529,7 @@
   no changes found
   devel-peer-request: getbundle
   devel-peer-request:   bookmarks: 1 bytes
-  devel-peer-request:   bundlecaps: 270 bytes
+  devel-peer-request:   bundlecaps: 275 bytes
   devel-peer-request:   cg: 1 bytes
   devel-peer-request:   common: 122 bytes
   devel-peer-request:   heads: 122 bytes