ci: verify support for Apache Kafka 4.0 in the integration matrix

[wbarnha]

Description

Adds Apache Kafka 4.0 to faust's live-broker integration test matrix so the
project is continuously verified against a 4.x broker (KRaft-only, released
2025) alongside 3.x:

kafka-version: ['3.8.1', '4.0.0']

The integration job's existing KAFKA_CONTROLLER_QUORUM_VOTERS static-voter
config works unchanged on 4.0 (static voters are still supported), so only the
matrix entry is needed.

Verification (against a real Kafka 4.0.0 broker)

I ran the full tests/integration/broker suite against a locally-run
apache-kafka 4.0.0 broker (KRaft, metadata.version 4.0-IV3) — all pass:

test_offset_commit.py::test_does_not_commit_past_inflight_message  PASSED
test_offset_commit.py::test_commits_contiguous_acks                PASSED
test_smoke.py::test_aiokafka_roundtrip                             PASSED
test_smoke.py::test_confluent_roundtrip                            PASSED
4 passed

Both transport drivers (aiokafka and confluent-kafka) round-trip correctly, and
the offset-commit regression tests pass. No faust code changes are required
for Kafka 4 support — this PR turns "it happens to work" into a tested
guarantee.

Notes

• The integration job is advisory (continue-on-error: true, not in the
  required check gate), so a flaky broker leg never blocks merges.
• Kafka 4.0 requires Java 17+ on the broker side; that's the container's
  concern (the apache/kafka:4.0.0 image), not faust's.
• Depends on the offset-commit tests from Fix #606: never commit past an in-flight (unacked) message #707 for the two test_offset_commit
  cases; the aiokafka/confluent smoke tests (already on master via ci: add live-broker integration test harness (Kafka in CI) #706) also
  exercise 4.0 on their own.

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (3c18394) to head (a436396).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #714   +/-   ##
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  Coverage   94.15%   94.15%           
=======================================
  Files         104      104           
  Lines       11136    11136           
  Branches     1201     1201           
=======================================
  Hits        10485    10485           
  Misses        550      550           
  Partials      101      101           

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