Update dependency karafka to v2.1.8

[renovate]

This PR contains the following updates:

Package	Update	Change
karafka (source, changelog)	patch	2.1.7 -> 2.1.8

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Release Notes

karafka/karafka (karafka)

v2.1.8

Compare Source

• [Improvement] Introduce Karafka::BaseConsumer#used? method to indicate, that at least one invocation of #consume took or will take place. This can be used as a replacement to the non-direct messages.count check for shutdown and revocation to ensure, that the consumption took place or is taking place (in case of running LRJ).
• [Improvement] Make messages#to_a return copy of the underlying array to prevent scenarios, where the mutation impacts offset management.
• [Improvement] Mitigate a librdkafka cooperative-sticky rebalance crash issue.
• [Improvement] Provide ability to overwrite consumer_persistence per subscribed topic. This is mostly useful for plugins and extensions developers.
• [Fix] Fix a case where the performance tracker would crash in case of mutation of messages to an empty state.

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