Bump MailKit to 4.16.0

[imnasnainaec]

The vulnerability in prior versions is preventing backend checks from completing on any pr.

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This change is 

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 02c432c1-6b0c-4fe3-9884-7b0a88751535

📥 Commits

Reviewing files that changed from the base of the PR and between e363f47 and fb8700d.

📒 Files selected for processing (3)

• Backend/BackendFramework.csproj
• Backend/Services/EmailService.cs
• docs/user_guide/assets/licenses/backend_licenses.txt

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch backend-bump-mailkit

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.94%. Comparing base (e363f47) to head (fb8700d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4267      +/-   ##
==========================================
+ Coverage   66.79%   75.94%   +9.14%     
==========================================
  Files         246      303      +57     
  Lines        6274    11352    +5078     
  Branches      787     1403     +616     
==========================================
+ Hits         4191     8621    +4430     
- Misses       1824     2330     +506     
- Partials      259      401     +142     

Flag	Coverage Δ
backend	87.23% <ø> (?)
frontend	66.79% <ø> (ø)

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[jasonleenaylor]

@jasonleenaylor reviewed 3 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on imnasnainaec).