Fixed #13417 - Added mailgun endpoint option, defaulting to the US

[snipe]

This just adds the mailgun endpoint to the services config file (defaulting to the US region so we don't break existing integrations), allowing UK mailgun users to add MAILGUN_ENDPOINT=api.eu.mailgun.net to their env per the mailgun and laravel documentation.

[what-the-diff]

PR Summary

• Addition of new 'endpoint' element in configuration
  The config/services.php file now includes a new element named endpoint within the mailgun array. It's pre-set with a value of 'api.mailgun.net'. This update enhances the flexibility of the email service, allowing it to use multiple endpoints.