PHP 8.0: Required parameters following optional parameters

In the Rules integration there are several examples of required parameters following optional parameters in function declaration. This is deprecated in PHP 8.0. Now that Drupal core supports PHP 8.0, and now that both Entity API and Rules have releases that are compatible with PHP 8.0, we can begin to deal with these issues in Mime Mail.

These instances of required parameters following optional parameters can easily be fixed by removing the default values for parameters that are before parameters that do not have defaults.