Add a CLI command airflow config update to update the config changes from airflow 2.x to 3.

[sunank200]

This PR introduces the airflow config update CLI command to help users transition smoothly from Airflow 2.x to 3. This command scans the current airflow.cfg file , then updates configuration parameters that have been removed, renamed, or had their default values changed. In addition, it cleans out deprecated configuration values (as updated in #47761 that have been emitting warnings in Airflow 2.x. For each such section-option:

• Renamed: Options that have been renamed (or moved across sections) are written under their new names with a comment indicating their original location.
• Removed Options: Deprecated options are omitted from the updated file.
• Default Value Updates: When an option’s value is updated to a new default (or if the effective value equals the default), the option is written as a commented line. A comment is added - for example:

  # Default updated from OldDefault to NewDefault
  # default_key = NewDefault

In addition, a backup of the original airflow.cfg is created before updating.

Examples:

• Renamed Option:
  A legacy key from the [admin] section:

  hide_sensitive_variable_fields = True

  is moved and renamed to core section:

  # Renamed from admin.hide_sensitive_variable_fields
  # hide_sensitive_var_conn_fields = True

  in the [core] section.

• Removed Option:
  Deprecated options such as

  check_slas = True

  in [core] are omitted entirely from the new configuration.

• Default Value Update:
  If an option in [core] is set to “OldDefault” (e.g. default_key = OldDefault) and the new default is “NewDefault”, the updated file will show as comment:

  [core]
  # Default updated from OldDefault to NewDefault
  # default_key = NewDefault

Usage Examples:

• Update the Entire Configuration:

  airflow config update

• Update Specific Sections:

  airflow config update --section core,database

• Update Specific Options:

  airflow config update --option sql_alchemy_conn,catchup_by_default

• Ignore Specific Sections/Options:

  airflow config update --ignore-section webserver --ignore-option check_slas

• Dry run for dummy detailed output:

  airflow config update --dry-run

closes: [#48376]

Tested locally:

• old airflow.cfg

• After running airflow config update:

• Also added the option to dry run:

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[Lee-W]

Mostly looks good to me. Left some nitpicks