[Snyk] Security upgrade nodemailer from 6.4.2 to 6.4.16

merge advice

:sparkles: Snyk has automatically assigned this pull request, set who gets assigned.

Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
- backend/package.json
- backend/package-lock.json

Severity	Priority Score (*)	Issue	Breaking Change	Exploit Maturity
	823/1000 Why? Proof of Concept exploit, Recently disclosed, Has a fix available, CVSS 8.6	Command Injection SNYK-JS-NODEMAILER-1038834	No	Proof of Concept

(*) Note that the real score may have changed since the PR was raised.

Commit messages

Package name: nodemailer

The new version differs by 34 commits.

ba31c64 v6.4.16
7e7b2b2 v6.4.15
fca2041 Update CHANGELOG.md
b4ccfa3 Oups
24b93bf Add ethereal.email to well-known/services.json
0f132fa doc: make the code a little more accessible with some code comments.
1815bad v6.4.14
dd26ddd v6.4.13
455cfbe v6.4.12
1787f22 Includes all information from the oath2 error response in the error message (#1191)
e3055c4 v6.4.11
3cfbf60 Create pull_request_template.md
35f9e33 Create CODE_OF_CONDUCT.md
8985bde v6.4.10
a0cab71 v6.4.8
5874437 Updated testable node versions
643c039 v6.4.7
b039ca5 Force charset for Content-Type headers even if not needed
5702a71 catch errors from invalid crypto.sign
5e00fb5 Update issue templates
c32431d Delete ISSUE_TEMPLATE.md
c034548 v6.4.6
3bfc545 fix: `requeueAttempts=n` should requeue `n` times
c73bb33 v6.4.5

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CodeChefVIT / FFDS