feature #42696 [Notifier] Mark Transport as final (fabpot)

This PR was merged into the 5.4 branch.

Discussion
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[Notifier] Mark Transport as final

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | n/a <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT
| Doc PR        | n/a

We've done the same for the Transport class of Mailer.

Commits
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3c38b5b037 [Notifier] Mark Transport as final

Author: fabpot

Transport.php

@@ -58,6 +58,8 @@
 
 /**
  * @author Fabien Potencier <fabien@symfony.com>
+ *
+ * @final since Symfony 5.4
  */
 class Transport
 {