Update phpmailer/phpmailer requirement from 6.9.3 to 6.10.0 in /lib (#7541)

Updates the requirements on [phpmailer/phpmailer](https://github.com/PHPMailer/PHPMailer) to permit the latest version.
- [Release notes](https://github.com/PHPMailer/PHPMailer/releases)
- [Changelog](https://github.com/PHPMailer/PHPMailer/blob/master/changelog.md)
- [Commits](https://github.com/PHPMailer/PHPMailer/compare/v6.9.3...v6.10.0)

---
updated-dependencies:
- dependency-name: phpmailer/phpmailer
  dependency-version: 6.10.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

lib/composer.json

@@ -13,7 +13,7 @@
     "require": {
         "marienfressinaud/lib_opml": "0.5.1",
         "phpgt/cssxpath": "v1.3.0",
-        "phpmailer/phpmailer": "6.9.3",
+        "phpmailer/phpmailer": "6.10.0",
         "simplepie/simplepie": "dev-freshrss#2e668d7b1bc2787310b99d2fd59bfda96e31b6ae"
     },
     "config": {