refactor: bump mailgun.js from 3.5.9 to 12.3.1

Bumps [mailgun.js](https://github.com/mailgun/mailgun.js) from 3.5.9 to 12.3.1.
- [Release notes](https://github.com/mailgun/mailgun.js/releases)
- [Changelog](https://github.com/mailgun/mailgun.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mailgun/mailgun.js/commits)

---
updated-dependencies:
- dependency-name: mailgun.js
  dependency-version: 12.3.1
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]