[FLINK-25538][Connectors/Kafka] Migration flink-connector-kafka from Junit4 to Junit5

[morazow]

What is the purpose of the change

• [JUnit5 Migration] Module: flink-connector-kafka, FLINK-25538.

Brief change log

• Updated Junit4 test packages to Junit5 test packages.
• This pull request is continuation of the previous pull request #66.

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)