Make classic_queue_consumer_unsent_message_limit configurable (backport #11857) (backport #11859)

[mergify]

Similar to other RabbitMQ internal credit flow configurations such as credit_flow_default_credit and msg_store_credit_disc_bound, this commit makes the classic_queue_consumer_unsent_message_limit configurable via advanced.config.

This PR is an alternative to #11822

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This is an automatic backport of pull request #11857 done by Mergify.

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This is an automatic backport of pull request #11859 done by Mergify.

[mergify]

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	both modified:   deps/rabbit/src/rabbit_queue_consumers.erl

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