[FLINK-27442][Formats][Avro Confluent] Add Confluent repo to module flink-sql-avro-confluent-registry

[MartijnVisser]

What is the purpose of the change

flink-sql-avro-confluent-registry depends on org.apache.kafka:kafka-clients, which is not available in Maven Central, but only in the Confluent repo. However, it does not configure this repo. This causes the build to fail for users locally.

Brief change log

• Added Confluent repo for flink-sql-avro-confluent-registry

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• 583bbd2 Azure: SUCCESS

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