Follow-up of SERVER-129905

Problem

The check_uuids_consistent_across_cluster hook always succeeds without performing any checks on release branches. Around MongoDB 5.0, config.chunks became keyed by uuid instead of ns (SERVER-53105,SERVER-59052), but the hook still aggregates with $group: {_id: "$ns", ...}. Since chunks documents no longer have an ns field, the aggregation returns no documents and the hook is effectively a no-op.

Fix

Update the aggregation to group by uuid so the hook validates UUID consistency across the cluster again.

Background

On master, the hook was removed instead (commit 728e242d, SERVER-129905), since checkMetadataConsistency already covers these checks and more (this is because on master we don't have to check config.cache because authoritative sharding is always used). For release branches, the decision is to fix the hook, so that we can keep the config.cache check.