Update confluentinc/cp-schema-registry Docker tag to v7.8.0

This MR contains the following updates:

Package	Type	Update	Change
confluentinc/cp-schema-registry	ironbank-docker	minor	7.7.1 -> 7.8.0
confluentinc/cp-schema-registry	stage	minor	7.7.1 -> 7.8.0

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