flagger_notify.settings must specify langcode because it contains translatable values

Problem/Motivation

The flagger_notify.settings configuration contains translatable values, but the
config object does not specify a language code. Drupal config inspection reports this as a validation error.

--------------------------- --------- ------------- ------------------------------------------------------------------------------------ 
  Key                         Status    Validatable   Data                                                                                
 --------------------------- --------- ------------- ------------------------------------------------------------------------------------ 
  flagger_notify.settings     Correct   89%           1 errors                                                                            
   flagger_notify.settings:   Correct   Validatable   The flagger_notify.settings config object must specify a language code, because it  
                                                      contains translatable values.                                                       
 --------------------------- --------- ------------- ------------------------------------------------------------------------------------ 

Steps to reproduce

1. Install Drupal 11.
2. Install the Flagger Notify module.
3. Run: drush config:inspect --detail --only-error

Proposed resolution

Add langcode to the default flagger_notify.settings configuration.

Example: langcode: en