Rename kubernetes config section to kubernetes_executor

[dstandish]

Now that KPO does not consider any core k8s config params, this section truly is just about kubernetes executor. Renaming it reduces potential for confusion.

[tanvn]

Hi, today I tried upgrading to 2.5.0 with helm-chart version 1.7.0 and found the following warning messages

> (app-root) bash-4.2$ airflow version
/opt/app-root/lib64/python3.8/site-packages/airflow/cli/cli_parser.py:905 DeprecationWarning: The namespace option in [kubernetes] has been moved to the namespace option in [kubernetes_executor] - the old setting has been used, but please update your config.
/opt/rh/rh-python38/root/usr/lib64/python3.8/configparser.py:408 FutureWarning: The config section [kubernetes] has been renamed to [kubernetes_executor]. Please update your `conf.get*` call to use the new name
2.5.0

I think we have to update this part:
https://github.com/apache/airflow/blob/main/chart/values.yaml#L1731

to change from kubernetes to kubernetes_executor as well, WDYT? @dstandish @uranusjr @potiuk

[dstandish]

Bad warning resolved here #28109

But yes there may be chart updates that need to happen

Make pr?

[tanvn]

@dstandish
Thank for the information.
I understood.
Let me create a PR for this later.