[Security] Bump phpmailer/phpmailer from 5.2.28 to 6.1.6

Bumps [phpmailer/phpmailer](https://github.com/PHPMailer/PHPMailer) from 5.2.28 to 6.1.6. **This update includes a security fix.**
- [Release notes](https://github.com/PHPMailer/PHPMailer/releases)
- [Changelog](https://github.com/PHPMailer/PHPMailer/blob/master/changelog.md)
- [Commits](PHPMailer/PHPMailer@v5.2.28...v6.1.6)

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

Author: dependabot-preview[bot]

composer.json

@@ -96,7 +96,7 @@
         "nstapelbroek/culpa": "^2.0",
         "pda/pheanstalk": "^3.1",
         "php-http/guzzle6-adapter": "^1.1",
-        "phpmailer/phpmailer": "~5.2",
+        "phpmailer/phpmailer": "~6.1",
         "phpoffice/phpspreadsheet": "^1.1",
         "predis/predis": "^1.1",
         "prettus/l5-repository": "^2.6",