Migrate zoekt_shards to zoekt_nodes

What does this MR do and why?

Migrate zoekt_shards to zoekt_nodes. Note that there is only one zoekt_shards record in production. After this MR is merged, we will repurpose zoekt_shards with a different schema to actually reflect a "zoekt shard".

This MR also removes the zoekt_shard_id NOT NULL constraint from zoekt_indexed_namespaces table as it is no longer used.

Related to #430787 (closed) (formerly #424456 (closed))

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