KIP-384 — Add config for incompatible changes to persistent metadata

Discussion Broker

Proposes a separate `metadata.version` (or similar) config distinct from `inter.broker.protocol.version` to control when brokers begin writing incompatible persistent metadata formats (e.g., `__consumer_offsets` schema). Using `inter.broker.protocol.version` alone for this purpose prevents safe downgrade once the new schema has been written, conflating wire protocol versioning with storage format versioning.

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