chore: bump kafka-clients to 4.1.0

[sebastian-alfers]

Upgrade the Kafka client library to 4.1.0

• https://kafka.apache.org/41/documentation/

[johanandren]

No need to bump test containers because client 4.0.0 also works with old brokers?

[sebastian-alfers]

test containers is still the version we have in main, isn't it?

[sebastian-alfers]

I think I misread your questions, which was more of a "if it should be bumped or not". I think not, since it works?

[johanandren]

Let's hold this off if bumping to 3.9 is enough for now.

[ennru]

I removed the Scala Steward restriction, should be OK to move along on the 4.x line.

[ennru]

The Mockito based tests need a review.

[ennru]

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