Update confluentinc/cp-kafka-rest Docker tag to v8

This MR contains the following updates:

Package	Type	Update	Change
confluentinc/cp-kafka-rest		major	7.7.2 -> 8.0.0
confluentinc/cp-kafka-rest	ironbank-docker	major	7.7.2 -> 8.0.0
confluentinc/cp-kafka-rest	stage	major	7.7.2 -> 8.0.0

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