fix(examples): sync exactly_once and consumergroup

[dnwe]

• ensure both examples handle sarama.ErrClosedConsumerGroup in the Consume for { ... } loop and ignore that error as it means the user has closed the ConsumerGroup directly via Close().
• ensure exactly_once handles the Messages() channel being closed with an ok check.
• copy assignment strategy optional flag from consumergroup into exactly_once sample
• bump the go.mod of both to specify 1.40.1 by default (although we still replace to ../.. anyway)