fix: class-not-found-error in Mailer

[Sarrus1]

Description

Rename the mail folder to Mail.

Motivation and Context

On Linux (and particularly in a Linux based Docker image), namespaces are case-sensitive. This caused the Mail class to not be found when trying to send an email (when restoring a password for instance).

How Has This Been Tested?

✅ Tested in a Docker container running the php:8.1-apache image.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.