Add Cassandra 6.0 support

[michaelsembwever]

https://issues.apache.org/jira/browse/CASSANALYTICS-37

Cassandra Analytics reads, bulk writes and consumes CDC through one bridge jar per major version. This adds the SIXZERO bridge, pinned to 6.0-alpha2 from Maven Central, in five new modules that copy and override the five-zero and four-zero sources.

Four 6.0 changes needed work:

• SSTable versions big-pa and bti-ea, both added for compression dictionary metadata in the CompressionInfo component.
• A trailing dictionary section in CompressionInfo. The six-zero CompressionMetadata deserializes it, interns one dictionary per dictId so that readers of one SSTable share the zstd compressor that ZstdDictionaryCompressor keys by dictId, and releases the references it holds through close(). SSTableCache gained a removal listener to call close(), and SSTableReader holds a second reference for the life of a stream.
• CEP-21 Transactional Cluster Metadata retypes Schema.instance to SchemaProvider and replaces transform() with submit(), which notifies no listener offline, so SchemaUpdater initializes the keyspace instances by hand after ClusterMetadataService.initializeForClients().
• Commit log version 9 and messaging version 14, read through the shaded 6.0 deserializers.

Two defects in the existing bridges surfaced while testing the copies and are fixed here. CassandraSchema.unregisterNonCdcTable called SchemaUpdater.load, which on 5.0 only adds a keyspace and throws AlreadyExistsException for one that exists, so the table stayed registered; removeTables replaces the keyspace metadata instead. On 5.0 that commit reaches Keyspace.dropCf, which needs machinery a client-mode process never started, so removeTables clears the initialized flag for the commit only.

BridgeVersionConsistencyTest asserts that every type a bridge hands out reports the bridge's own version, which catches a six-zero module that silently inherits a version-bearing class from five-zero through the Gradle copy-forward.