[FLINK-25189][connectors/elasticsearch] Update Elasticsearch sinks to latest minor versions

[alpreu]

What is the purpose of the change

We want to update the elasticsearch sink connectors to use the latest available minor version for Elasticsearch 6 and 7.

Brief change log

• flink-connector-elasticsearch-base and flink-connector-elasticsearch-7 have been updated to Elasticsearch v7.15.2
• flink-connector-elasticsearch-6 has been updated to Elasticsearch v6.8.20
• Some classes (e.g. TimeValue) have been relocated by Elastic in between ES7 versions, therefore the ElasticsearchApiCallBridge for the old interface as well as the ElasticsearchXSinkBuilder for the new interfaces had to be changed to accommodate the different module locations of the class.
• correct the connector docs to state that foe each major versions the connector is compatible across all minor versions

Verifying this change

This change is already covered by the existing tests, such as the modules unit tests and e2e tests.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

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CI report:

• e26b6a0 Azure: SUCCESS

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Automated Checks

Last check on commit 84c2b41 (Tue Dec 07 09:14:18 UTC 2021)

Warnings:

• 5 pom.xml files were touched: Check for build and licensing issues.
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[fapaul]

Thanks for doing the cleanup. Looks good but I left some minor questions.