Update quickstars/email-users

[PaulRBerg]

The last version(2.4.1) of nodemailer is officially deprecated, so I added 4.0.1.

Did this to ensure the functionality. Furthermore, tried to run the project, but didn't work until I altered the firebase.json file by changing the path from "public" to "./public". Otherwise, the hosting website wouldn't be found.

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[PaulRBerg]

Signed the CLA.

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CLAs look good, thanks!

[nicolasgarnier]

Thanks @PaulRBerg