Bump Cassandra and Elasticsearch images & add persistent volumes

[dblugeon]

Hello JanusGraph Team,

This pull request contains some enhancements for the example of compose file.

• An update Cassandra image to 4.0.20
• An update Elasticsearch image to 8.19.15
• Increase Cassandra health‑check retries. On slow disks, Cassandra could incorrectly be marked unhealthy with the previous settings
• Add JCE_ELASTICSEARCH_DATA and JCE_CASSANDRA_DATA compose environment variables. Use docker volumes janusgraph-cassandra-data and janusgraph-elasticsearch-data by default

Regards

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