feature #35519 [Mailer] Make default factories public (fabpot)

This PR was merged into the 5.1-dev branch.

Discussion
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[Mailer] Make default factories public

| Q             | A
| ------------- | ---
| Branch?       | master <!-- see below -->
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #35469 <!-- prefix each issue number with "Fix #", if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

To allow registering new factories and keep the default ones available.

Commits
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c9863c6 [Mailer] Make default factories public

src/Symfony/Component/Mailer/Transport.php

@@ -149,7 +149,7 @@ public function fromDsnObject(Dsn $dsn): TransportInterface
         throw new UnsupportedSchemeException($dsn);
     }
 
-    private static function getDefaultFactories(EventDispatcherInterface $dispatcher = null, HttpClientInterface $client = null, LoggerInterface $logger = null): iterable
+    public static function getDefaultFactories(EventDispatcherInterface $dispatcher = null, HttpClientInterface $client = null, LoggerInterface $logger = null): iterable
     {
         foreach (self::FACTORY_CLASSES as $factoryClass) {
             if (class_exists($factoryClass)) {