chore: update dskit to include LRU cache for partition ring shuffle sharding

[Segflow]

What this PR does

This PR updates the dskit dependency to version v0.0.0-20251218155159-03c38747d0d9 which includes the new LRU cache feature for partition ring shuffle sharding.

Why

The updated dskit version includes PR grafana/dskit#856 which adds optional LRU-based caching to the partition ring shuffle shard cache. This is needed for the DataObj consumer to efficiently manage shard caching with bounded memory usage.

Changes

• Updated go.mod to use dskit commit 03c38747d0d9584b1f537158f0fa90aabefc5142
• Updated test files to handle the breaking change in ring.NewPartitionRing() which now returns (ring, error) instead of just ring
• Updated vendor directory with the new dskit version

Related

• feat: add LRU cache to partition ring shuffle shard cache dskit#856 - Add LRU cache to partition ring shuffle shard cache