Stop testing Kafka against end-of-life Confluent Platform 7.5

[cristianrgreco]

Summary

Part of bringing existing modules in line with the supported-versions rule proposed in #1587 (other batches: #1589, #1591): a module's tests cover the latest release of every major version the vendor still supports.

Kafka already follows the shape of that rule: the Dockerfile image (Confluent Platform 8.3) runs kafka-container-latest.test.ts, and major 7 has its own file because it needs ZooKeeper tests the 8.x suite does not have. What was wrong is which 7.x images that file used:

• kafka-container-7.test.ts now runs on cp-kafka:7.9.10, the latest 7.x release (was 7.9.1). Community support for 7.9 ends 2027-02-19 (Supported Versions and Interoperability).
• It no longer starts containers from a line past end of life: the two SASL tests on cp-kafka:7.5.0 and the ZooKeeper container on cp-zookeeper:7.5.0 now use 7.9.10. Confluent's table no longer lists 7.5, and 7.6 ended 2026-02-09.

Redpanda has a single supported major that is not about to end (25.3 ends 2026-11-19), so its tests stay on the Dockerfile image and are not changed.

Code for the ended Kafka versions is removed separately in #1597.

Verification

Local, macOS arm64, Docker Desktop:

Run	Result
npx vitest run packages/modules/kafka/src/kafka-container-7.test.ts --maxConcurrency=2	12 passed, 58 s

• npm run format, npm run lint, npm run knip: clean.
• The docs snippets connectBuiltInZK, kafkaSsl, connectProvidedZK and connectKraft keep their markers; kafkaSsl now shows new KafkaContainer(IMAGE).

Left to CI: Podman, Node 22 and 24, linux/amd64, and the docs build.

Test count unchanged (17). Images: cp-kafka:7.9.10 and cp-zookeeper:7.9.10 replace 7.9.1 and the two 7.5.0 images.

An earlier revision of this PR also ran the 8.x suite on cp-kafka:8.2.4. That overloaded the CI runner (broker transport failure, passing only on retries) and is gone now that only one image per major is listed.

Not breaking

One test file. No module source, exports or Dockerfile changed.