Bug fixes

Bug fix

Switched to Mailgun SDK ~3.0, added EU support, made Drupal 9 compatible and fixed many issues.

If you have difficulties upgrading to this release by running composer update --with-dependencies try the following approach:
composer remove drupal/mailgun (to remove the module will all dependencies)
composer require drupal/mailgun (to install it from scratch)

Fix a regression we introduced that the upgrade path didn't run on D8.7 anymore.