Improve executor validation in CLI

[mik-laj]

Previously, when a user tried to check configurations for celery, they got a message that the celery SUBCOMMAND is not supported. This is confusing as user was running config get-value and not celery *.

airflow config get-value celery broker-url
usage: airflow config get-value [-h] section option

Print the value of the configuration

positional arguments:
  section     The section name
  option      The option name

optional arguments:
  -h, --help  show this help message and exit

airflow config get-value command error: argument section: celery subcommand works only with CeleryExecutor, your current executor: LocalExecutor, see help above.

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[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.