Update confluentinc/cp-kafka-rest Docker tag to v8.1.1

This MR contains the following updates:

Package	Type	Update	Change
confluentinc/cp-kafka-rest		minor	8.0.2-1-ubi9 -> 8.1.1-1-ubi9
confluentinc/cp-kafka-rest	ironbank-docker	minor	8.0.2 -> 8.1.1
confluentinc/cp-kafka-rest	stage	minor	8.0.2 -> 8.1.1

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