fix: lower max msg byte in docker to allow us to test against kafka 4.0

[petedannemann]

Kafka 4.0 currently runs out of memory when a high max message size is used, lowering this allows us to test against Kafka 4.0 in CI https://github.com/segmentio/kafka-go/issues/1360#issuecomment-3004986585

For consistency I have done this everywhere for all Kafka versions we test against. It was probably a mistake to make it so large from the start

[dajac]

If it is not enough, you can try to reduce it further (e.g. 1MB) and you can also reduce the number of partitions with offsets.topic.num.partitions. It is 50 by default. 5 may be enough for your tests.