catalog: stop checking the upgrade shard

[teskje]

The upgrade shard was introduced because opening the catalog for reading would automatically bump the version of the catalog shard, possibly fencing out existing readers unintentionally. This doesn't happen anymore, persist shard versions now must be bumped explicitly. As a result, we don't need the upgrade shard anymore and can stop checking it.

This still leaves around an initial version check during catalog open (using the catalog shard version). This isn't required for correctness but ensures that we get a nice error rather than panicking in persist. Makes tests easier to write, though it is something we could remove if it ends up being inconvenient.

[github-actions]

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