Update cassandra
Update cassandra

Changes:

- https://github.com/docker-library/cassandra/commit/8b1427f: Merge pull request https://github.com/docker-library/cassandra/pull/217 from infosiftr/focal
- https://github.com/docker-library/cassandra/commit/ac67762: Update to Ubuntu Focal
- https://github.com/docker-library/cassandra/commit/871b6f0: Fix .gitattributes specificity

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