Make MailManager::doMail protected

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This could use an issue summary update as it was suggested this method be deprecated.

   public function doMail($module, $key, $to, $langcode, $params = [], $reply = NULL, $send = TRUE) {
+    $backtrace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 2);
+    if ($backtrace[1]['class'] != 'Drupal\Core\Mail\MailManager') {
+      @trigger_error('Calling' . __METHOD__ . '() directly is deprecated in Drupal 10.1.0 and it will be marked as protected in Drupal 11.0.0. See https://www.drupal.org/node/NID', E_USER_DEPRECATED);
+    }

Deprecate it like the above?

Not sure if it needs to be wrapped in anything. If we are deprecating the whole method it could brought be the first line of the method. Would need a test case also.

#23 is deprecating the public access to make it protect later which is the scope as the title.