[hotfix][docs] Pull request to update documentation for Flink 2.0 support.

[och5351]

• Fill out the template below to describe the changes contributed by the pull request. That will give reviewers the context they need to do the review.

What is the purpose of the change

In Flink Kubernetes Operator version 1.12, the contents overview does not specify support for Flink 2.0. Therefore, I have added v2_0 to the doc files at the following two paths.

• /flink-kubernetes-operator/docs/content/docs/concepts/overview.md
• /flink-kubernetes-operator/docs/content.zh/docs/concepts/overview.md

Brief change log

> when playing 2.1 version error message > flink kubernetes operator 1.12 spec FlinkVersion code

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 changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: no

Documentation

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