[hotfix] [docs] Fix flink-connector-kafka version in "How to use Maven.." doc of Flink master

[Prashant15887]

The "Using Maven" page in Flink's latest release documentation populates version of flink-connector-kafka in an example using Flink's version initialized in the config.toml file. As Flink version is set as 2.1-SNAPSHOT in config.toml, the version of flink-connector-kafka also gets populated as the same. However, checking flink-connector-kafka's repository, there is no 2.1-SNAPSHOT there. This causes the mvn build to break at the doc reader's end. To avoid the problem, either we can initialize a new dedicated version variable for flink-connector-kafka in config.toml and use it in this doc page. Or, if it looks like too much of a change for one mention, can we populate an existing flink-connector-kafka version in the doc example so that the reader's build does not break.

https://repository.apache.org/content/repositories/snapshots/org/apache/flink/flink-connector-kafka/

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Contribution Checklist

• Make sure that the pull request corresponds to a JIRA issue. Exceptions are made for typos in JavaDoc or documentation files, which need no JIRA issue.
  A- This pertains to one minor change in the Flink docs. There is no JIRA issue created for it hence.

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(The sections below can be removed for hotfixes of typos)

What is the purpose of the change

The version of Flink Connector Kafka given in Flink doc master causes MVN build to fail. This is because the doc reuses the latest version of Flink to populate the Flink Connector Kafka version also. The version given in the doc does not exist in the Flink Connector Kafka repository causing the build to fail. The purpose of this change is to update the Flink Connector Kafka version in the doc to an existing one.

Brief change log

Replace the Flink version variable (initialized in config.toml file) used in maven.md doc with actual existing version of Flink Connector Kafka.

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change is a trivial rework / code cleanup without any test coverage.
The CI build results are shared below.

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

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CI report:

• bbc8103 Azure: SUCCESS

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