Bump swiftmailer/swiftmailer from 5.4.8 to 6.2.3

Bumps [swiftmailer/swiftmailer](https://github.com/swiftmailer/swiftmailer) from 5.4.8 to 6.2.3.
- [Release notes](https://github.com/swiftmailer/swiftmailer/releases)
- [Changelog](https://github.com/swiftmailer/swiftmailer/blob/master/CHANGES)
- [Commits](https://github.com/swiftmailer/swiftmailer/compare/v5.4.8...v6.2.3)

Signed-off-by: dependabot[bot] <support@github.com>

composer.json

@@ -31,7 +31,7 @@
         "erusev/parsedown" : "^1.7",
         "pimple/pimple" : "3.2.2",
         "psr/log": "~1.0",
-        "swiftmailer/swiftmailer" : "5.4.8",
+        "swiftmailer/swiftmailer" : "6.2.3",
         "symfony/console" : "4.2.12",
         "symfony/event-dispatcher" : "3.4.2",
         "gregwar/captcha": "1.1.8"