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-lfs-serve.t	Mon Feb 27 19:59:55 2023 -0500
+++ b/tests/test-lfs-serve.t	Mon Feb 27 23:04:09 2023 -0500
@@ -308,9 +308,13 @@
   $ hg -R $TESTTMP/client4_pull pull http://localhost:$HGPORT
   pulling from http://localhost:$HGPORT/
   requesting all changes
-  remote: abort: no common changegroup version
-  abort: pull failed on remote
-  [100]
+  adding changesets
+  adding manifests
+  adding file changes
+  transaction abort!
+  rollback completed
+  abort: missing processor for flag '0x2000'
+  [50]
   $ hg debugrequires -R $TESTTMP/client4_pull/ | grep 'lfs'
   [1]
   $ hg debugrequires -R $SERVER_PATH --config extensions.lfs= | grep 'lfs'