[FLINK-35140][docs] Update Opensearch connector docs for 1.2.0 and 2.0.0 releases

[snuyanzin]

What is the purpose of the change

Add docs for new Opensearch connector release In fact docs are same for both v1 and v2, so just referencing to one branch (also references to multiple branches are not supported here IIRC)

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): (yes / no / don't know)
• 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? (docs)

[flinkbot]

CI report:

• a674f58ce2d5d444181a82a6383e7e738fd52bb4 Azure: SUCCESS

Bot commands

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[snuyanzin]

@flinkbot run azure